Vermicast (or "VC" if you're into the whole brevity thing) can seem like a bit of a conundrum in the literature. It is at once described as a potent "fertilizer" with amazing properties, but in the same breath we're told that it can be used in any quantity and it cannot burn plants like a fertilizer can. Both of these claims are perfectly true! At issue is the term "fertilizer". VC isn't really a fertilizer in the way that we typically think of fertilizer, as a supplement that adds things like nitrogen/phosphorous/potassium and/or minerals to a plant's growth medium. Vermicast does of course have these nutrients, but its value isn't in the NPK it adds. What it really does is add so many other elements like plant growth hormones, humates to stabilize and facilitate growth, beneficial microbes and fungi (who are the ones who actually fertilize plants in a healthy soil system), adding structure and moisture retention and more!
Vermicast can be used for starting seedlings at up to 100%, but in usual soil and potting mixes (for containers) it is most effective added at 10%-20% by volume of the final mix. It can be added as a top-dressing to existing plants and container plants, and is most effective if stirred in lightly and/or mulched so the microbes are protected until they can work into the soil.
This is one of the really exciting developing opportunities for getting some amazing benefits out of your vermicompost. Actively Aerated Compost Tea (AACT) has long been known to provide remarkable benefits to plants and to soil. AACT can revitalize soils that are bereft of living microbes, add nutrition to soils even if they are in good shape, directly feed and inoculate plants' roots (rhizosphere culture), increase resistance to pests, and even be used foliarly as a feed or to prevent and even treat disease like mildew, blights and more.
Since it is essentially a super-charge of soil ecosystem microbes and fungi that can simply be poured/applied, it can also be used to start a compost pile, treat lawns and turf, it has so many uses. One of my regular uses is to revitalize the soil in container plants... since they often get totally dried out and/or sit in the sun, container plants' microbial communities in their soils can be a lot less robust. This is why synthetic fertilizers are so popular with houseplants: their microbial community is weak and has difficulty processing organic matter into substances that plants can take up properly.
This is a HUGE topic so I will refer you to your own reading on AACT and its use, and to my instructions for making tea with the kits I sell.
Vermicast is great for top-dressing lawns, turf and ground covers. It can be spread using a regular rotary spreader and really cannot be over-applied, but something in the middle of the broadcast rate dial should be appropriate for most uses.
While vermicast has a lot of physical properties beneficial for soil structure, tilth and water-retention, you know by now that the reason it is such an amazing product is its living complement of microbial life. As such, it's important to store it in a manner that keeps that biota alive and healthy. The microbes and fungi in living vermicompost need the same things that they did in the worm bin: stable temperatures, moisture, oxygen, and protection from sunlight. Since their life is not at the actively booming stage that they are in a growing bin, they can be stored safely for some time as long as these basics are met.
For vermicomposters the single best place to store your vermicast is... in your worm bin! That may sound silly but hear me out. While it's naturally exciting to get your first harvests from your bin, consider that having an inventory of vermicompost in situ offers some advantages you may not realize. 1) It gives your worms super-important refuge areas to retreat to if something happens that makes a portion of the bin uncomfortable for them (heating, ammonia release, pH swings etc) until things settle down. 2) By keeping it along with the larger mass of the bin ecosystem, the vermicast and the whole bin experience much narrower swings in temperature and in moisture content. I'm not saying don't harvest your VC until the last second, far from it! But you also really don't need to be in a rush to harvest just for its own sake.
OK, so you've harvested and sifted a bunch of VC, or have some left over you haven't applied yet. Now what? If you're not using it right away, the best way I've found is to store it in plastic buckets with the lid left just barely ajar. By that I mean on top of the bucket but not fully snapped down. The biggest risk for us here on the Front Range is desiccation, where our dry air dries out the vermicast in storage which can harm the microbial community. They need oxygen but not a ton, so just a modest amount of gas exchange will keep them healthy and still allow respiration but without excess drying.