You progress as a Vampire depending upon your satiation and how many victims you have fed upon.
You determine if you wish to be more powerful when fully fed or starving.  You can control feeding! -----> each time you feed you add to your satiation and progress one stage.
Each feeding victim adds to your power and helps you advance through Vampire ranks.  If you want to, you can also choose to advance through ranks based on your days as a vampire.
Specific messages let you know what rank and stage of vampirism you have entered, and messages keep you informed of your satiation levels.
Stages last 24 hours in game, but if you enable Dynamic Vampire Stages you will have to feed more quickly as you starve for blood (STAGE 1 ---12hr---> STAGE 2 ---6hr---> STAGE 3 ---3hr---> STAGE 4). If you enable two stage feeding, you will either be fully fed (lasting 24hr) or starving.

You can choose to progress as a Vampire depending on the number of Blood Points you currently possess.  This option is ONLY available if you choose reverse progression.  Each time you feed you will receive Blood Points for your use, and they will fade slowly over time.  Use your blood pool wisely though ... because using your abilities will consume points and you'll progress faster toward being starved and weak.

You can choose to have Vampire Drain (and Drain Life and Vampiric Grasp if you are a Vampire Lord) affect your satiation.
You determine if you want using these spells to drain some blood away from your victim and nourish you.  If you drain enough away it can affect your stage of vampirism and blood points (if they are enabled).

Different stages of your vampirism will bring about new strengths, powers, and weaknesses. Stats and abilities scale with level categories: 1-19, 20-29, 30-39, 40-49, 50+

You have in-depth knowledge of your vampiric state and you will know when your blood reserves will deplete. A detailed description of your Vampiric Stage can be found under "Active Effects" - it also has a timer (real life time) to let you know when you will progress to the next stage.  I have also implemented a customizable hotkey (X by default) that lets you track your Vampire status – it requires SKSE.  A new fully customizable blood meter can display your blood levels if you wish. 

Long-term feeding on victims will enhance your vampirism and you will enjoy stronger attacks, increased defense, and all new additional abilities. Rank progression is determined by your experience in Skyrim and the number of victims you have drained of blood. You can choose to have ranks based on feeding victims and your level, or just the number of feeding victims.  You can also choose to have ranks based on your days as a vampire and have it independent of your feeding victims.

If you choose to have a hated stage (fully engorged, starving, or always) the citizens of Skyrim can immediately recognize you as a Vampire and attack without remorse. An illusion spell will temporarily hide your true form and remove all hate, but it is taxing to maintain and will wear off eventually.

You will be weaker in the light of day and much stronger at night or inside. You can choose to have true sun damage; it will behave realistically. If you choose to enable sun damage you may not take damage during really overcast weather or while in shadows - health and stamina will slowly regen as well if light levels are low enough.  You can also customize the light level threshold which determines if you are in shadows or not - useful if you use an ENB or have altered the vanilla light levels.

As a Vampire, you can feed on those who are asleep, paralyzed, seduced, bleeding out, or even those recently dead if you choose. If you want to feed on a dead body, you have to be crouching (sneaking) at the time.  A new option lets you sneak up behind an unwary victim and feed on them as an attack – they may die or they may escape your grasp. 
As a Vampire Lord, you can feed on those who are asleep, seduced, or bleeding out.  Each time you ingest blood it will restore a small amount of your health and stamina. When you feed on a person you will damage their health and eventually drain them of all of their blood and kill them if you overfeed (two feedings in a row usually does them in). If a NPC is set to be essential, they won't die, but a variable will be set on them to disallow further feeding.  If you wait 24 hours after feeding on an NPC, the health damage from your feeding will be fully restored. You have the option of having dead bodies help advance you through vampiric stages and ranks - or not.

Once you reach Nightlord Vampire rank, Amaranth can allow you to feed on other Vampires with a chance to absorb some skill from them. This will completely drain and kill the other Vampire; you will not be able to feed on those you have turned into Vampires.

In combat, if you have more health than your victim, you will be able to "Bite" a staggered opponent if you are within activation range. This will damage and drain away a portion of their health - giving it to you - and replenish some blood points (if you have this enabled).  It doesn't count as a feeding, but it does require some stamina to perform (like a special move).  Damage done increases as you level.

At your highest rank, you will be able to earn perk points in exchange for ongoing feeding. With normal progression, every 80 feeding victims will result in the addition of one perk point; 40 feeding victims are required with easier progression for each perk point. SKSE is required to add perk points.

At your highest rank, you will also be able to increase your total Health, Stamina, and Magicka with each feeding victim.  Each victim will add one point to each stat; feeding on vampires will add two points to each stat.  These stat bonuses will be removed if you are cured of vampirism (but will be reinstated if you infect yourself again).

This mod has its own method of extracting Blood Potions (if you enable it). You can extract blood from victims that are sleeping, paralyzed, knocked out, seduced, or even from corpses (if you choose). Blood Potions will heal you (health, stamina, and magicka) and perhaps satisfy your hunger (if you enable it), but they will not add to your number of victims for rank progression. If you extract blood from a victim twice in 24hr you will kill them – just like overfeeding will.  If you extract blood from a corpse, it will give you a weaker version called a Stale Blood Potion.  You need to be crouching (sneaking) to extract blood from corpses.

You can choose to make your feeding victim into a Vampire. Others that you bestow with the gift of immortality will willingly join you as a follower (if possible).  Those you turn into Vampires will have vampiric strengths/weaknesses and abilities.  Those you turn will feed when they hunger (can be disabled if you wish).

You can choose to make a helpless victim into your thrall (Vampire Cattle). Victims will willingly join you as a follower (if possible), or they will stand mesmerized in place - waiting for their master to feed on them.

You have inherited the Vampire's weakness to silver. It will burn your flesh and cause your blood to coagulate in your veins. All Vampires take additional damage from silver weapons: they burn for 5-20 points of damage depending on your level (i.e. 5 when <20, 10 when <40, 13 when <60, 17 when <80, and 20 when > 80)

If you should suffer an untimely demise, Tamriel will have nothing left but your ashes to remind them of your existence ... Your body will turn to an ash pile upon death - tragic yes, but befitting a Vampire. Turning to ash can be disabled if you wish.