From my understanding freesync/gsync only matters when you're dipping below the refresh rate of the monitor right? If I'm able to maintain FPS above the refresh rate I should be fine correct?

The reason I ask is because I purchased a RX580 for my 1440 100hz Freesync monitor with intents to return it and buy vega when it comes out, but now that release has been pushed back to July it will be a pain as I have to return the rx580 by the end of June. I'm wondering if it's worth it to just switch to Nvidia or if I'm going to have performance issues with the monitor.

You just won't be able to use the freesync

But no it does more than that

Sell the Rx 580 for lots of $$ on OfferUp or Craigslist.
The miners want them.

Then go buy a 1070 lol

My area doesn't do well for PC parts or else I'd consider it

If your area is out of stock, miners want them and might pay more than they are worth. It's something worth checking out imo