How bad is a bent rim, how to deal with it and how urgent and dangerous is it?
Riding a slightly bent rim is typically not dangerous. I've continued to ride bent rims on my bikes for many months. Eventually, it may get worse, especially if you are learning 180's or how to jump. The bend is a weak spot that can get worse if you land on it with the same side loads again. From your description, it sounds like it isn't a giant bend— you can only feel it vibrating at high speed, and it isn't bent enough to contact your frame or otherwise disrupt how your wheel rolls.

It could be dangerous if the rim is cracked at all. Usually a rim only cracks under extreme damage, or after months of riding it bent, and at that point becomes much weaker. A cracked rim could collapse under load, or especially harsh impacts like landing jumps. Inspect your rim again for cracks around the bend just to be sure.

Usually, a bike shop can fix your wheel. Most bicycle wheels consist of the rim, spokes, and a hub at the center, with a bearing and axle that connect to your frame. The spokes actually give your wheel most of its stability. It's similar to a suspension bridge, where the rim is not super strong by itself, but with tension from the spokes it becomes much stronger.

If your wheel is this type, a bike shop can "true" your wheel by tightening and loosening the spokes around the bend. They have special tools to help get your wheel back in true (straight) by making minor adjustments on the spokes throughout your wheel. Usually truing your wheel doesn't require new parts, it just requires labor, and should be under $50. Also, a good wheel true can make your wheel stronger, so you might have less problems with it in the future.

This does depend on the type of bike and wheel you have though. Cheaper rims can be harder to fix, and if its bent too badly to be trued, you may need a new wheel.

When dealing with the bike shop, simply show them your wheel and ask if it can be trued. If yes, they'll tell you a price and when they can do it. If not, you can ask them about new wheel options. Replacing just a rim with your old hub and spokes is very labor intensive, which is expensive. It sounds like your wheel should be truable. But if it is not, the shop will want to sell you a new wheel, because it simply isn't practical usually to replace just the rim.