I didn't take the voluntary furlough for reasons mentioned, big chunk of cash at 12 years. I didn't give them 2 weeks, but had dropped the trips earlier, still NK didn't pay out the 84 hours of vacation they owed me ( when I left 100s of pilots were leaving, and SPA-ALPA knew the company was using the 2 weeks notice to not cash out vacation, but they didn't advise the pilots AFAIK, not awesome). And it took half a year to get my sick bank paid. When I asked SPAMEC about the sick/vacation, they said they couldn't help me because I didn't work for NK. Even Carl was shocked how this was handled. Absolutely made me feel leaving was the right decision.
Don't voluntary furlough if it overlaps. Do give them 2 weeks if you want your vacation pay.

I’m kinda planning both. Taking the voluntary furlough to prevent an involuntary furlough for someone else, but also giving 2 weeks notice fore coming to UA.

You’d have to ask a lawyer but I don’t think not giving 2 weeks notice is grounds for denying payout of unused vacation. You’d may have a legal case against Spirit if you never got your money.

Sure. I didn't give 2 weeks notice at SWA, so I'm sure I'm blacklisted from ever working there again. But UAL won't care what kind of notice you gave, just as long as you're not employed at 2 carriers at the same time...

Preferably give them a day to process the resignation and not on a weekend.

You could in theory resign via email after midnight effective immediately (bcc your personal account) and show up to class at United after breakfast…