If you do it right, it won't need to be replaced. You're just making a silicone gasket rather than metal.

I'm taking it to a shop to have it installed. Should I attempt to do this or should I just wait for the gaskets to come in? That's even if the vendor will send me the damn things.

Just roll it up on some ramps and do it yourself. It's a cat back. If you can change your oil successfully you can do this. When you're buying rtv, buy some ramps and some pb blaster. Now you've got everything you need for less than you would have paid the shop.

As a rule-of-thumb, a gasket shouldn't be re-used once it has been compressed. Gaskets are cheap - get new ones.

If your used gaskets are in good shape (no cracks/separations/wrinkles/&c), you may be able to use them again. If you re-use the gaskets, I suggest putting some high-temp RTV (or other sealant) on both sides of the gasket to seal invisible cracks. Gaskets are cheap - get new ones.

The vendor shouldn't charge me for sending me the gaskets right?

If they come with the kit normally, then no they shouldn't charge you for the missing parts and ship them out on their own dime.

You already paid for them when you bought the exhaust.

True