Where would you ride early september within a days drive of Vegas?

Rampage. 

I'd check out deer valley and brian head in Utah. Likely possible to hit them both in one trip since deer valley is open daily in the first week of september iirc

Deer valley is more polished and has more variety but brian head is a bit of a gem if you like chunky trails.

Mammoth is probably your best bet.  Also Big Bear.

I wanted to suggest Richfield UT shuttles, weather-permitting, but it seems the trail area is out due to a large fire...

Damn I was just about to recommend Richfield, sucks about the fire.

It's more pedal oriented but St George / Hurricane has plenty of trail options. All of the Mesa's. Shouldn't be too hot?? (Only visited those places in the spring)

Bootleg canyon if you want to ride some gnar

Sedona if you want to do pedaling. 

not in september.

Yeah, bootleg climb road has no shade at all, and Sept is still very hot.  It can be shuttled though almost to the end of the climb, but then again you have to start super early when it might still be tolerable.

Re Rampage, most of the riding around the place seems to be an XC pedalfest with maybe a a few fun bits on the Mesas.  Notable exception is Flying Monkey / King Kong trail.  Also, I'd bet it will still be too hot to ride.

There's Snow Summit.  Probably better jump lines and possibly closer than Mammoth & Northstar, though driving on US395 is far more scenic and relaxing than sharing I-15 with road-raging drivers who blew their 401k at casinos.  Since you got 5 days, you could possibly ride all 3.

On Utah side, aside from everything mentioned thus far (really shame about Richfield)...  

Revenant trails up on a mountain west of St. George.  I haven't ridden there, but looks fun in videos I've seen and it's on your way to St.G via a slight detour.

Zion Mountain Ranch.  You have to pedal up quite a bit, but a couple of trails are pretty good - Duck Duck Goose sticks out in my mind, there was a jump line but only the first two features were built when I was there, I imagine it's completed by now.  

There's some good stuff in Cedar City (Boneyard, Blowhard, etc), it's worth extra 30-mile drive if you find yourself in Hurricane/St.G.  

Little bit further north, there's a trail called Thunder Mountain near Panguich.  This isn't a bike-optimized trail like others mentioned above, but it was reasonably fun to ride and highly recommended if you happen to enjoy long scenic adventure rides.

Lastly, there is a bike park just north of LV up on Lee Canyon.  There wasn't much to ride when I went there a few years ago right after it opened, but I was told there are some good trails these days (second-hand info).  Could be a good place to go if you happen to get an afternoon off and it's 100+ ºF in the valley.

I was curious how close all three of those bike parks were to Vegas, so I checked Google Maps and it says 3.5 hours to Snow Summit from Vegas, 5.5 hours to Mammoth, and 7.5 hours to Northstar. 213 miles, 312 miles, and 477 miles respectively. Although if you head NW, you could double up and Mammoth and Northstar, because Mammoth is more or less en route to Tahoe from Vegas, and that would be doable in a busy five day stretch. But they're not exactly close to Vegas.

Seriously if I was good enough to ride King Kong (or even Flying Monkey) that’d be the move. I think Virgin is only like 2.5 hours from Las Vegas and you could check out Zion and rent a Harley to ride with no helmet. Probably a place to shoot machine guns too. 
Go poke around the Old Rampage site which is the New Rampage site while you’re at it. Check out some fundamentalist Mormon church options.

I don’t really know how your trip looks where you’re in Vegas with your mountain bike. I guess you’re driving there from somewhere? And you’re making a loop of it or?

I was at Rampage a long time ago and I would go back to the area for a few things but bikes really aren’t necessary lol

But I’m definitely also not driving from Las Vegas to ride deer valley by myself holy shit

If you're still reading this, ignore if you want chair lifts:

Hadn't seen anyone mention Northern AZ yet.

Flagstaff AZ is 3.5 hours away from Vegas, and Mt Elden is super fun. Your choice of single track or road climbs. Plenty of good trails for less effort that can be looped or lapped, just as many if you pedal to the top. Fun college town and it's about 10 minutes from Elden. Weather is perfect in September.

Big ups on Mt. Elden; I rode there many years ago, but it was an absolute blast. The view from the top of Elden is pretty unreal as well and makes the climb worth it. 

This seems like the answer to me!

If you're going the flagstaff route but also want to hit a bike park, sunrise is three hours further east of flag and is a bit of gem imo