I have put 125,000 miles on two Vulcan 750s, a '93 and an '02, both bought new, and I have done a lot of spline lubes. I do them about every 10,000 miles. I have used various types of grease, from BelRay waterproof grease, which contains no moly, to the Guard Dog 73% stuff, and can't really tell a difference. I have never found any spline wear. I do not skimp on the grease, I pack everything full, on the back (I do both the front and rear splines) I completely fill the coupler, then push the final drive into it, forcing out all the excess grease. I clean up the mess next time, and do it again.
I think a big part of the problem to begin with is they never used enough grease to begin with at the factory, if they used any grease at all. On the ones they actually did anything to, I think they used a tiny amount of almost pure moly. IMO, pack those splines good, front and rear, and also the ones where the rear wheel engages the final drive. But be aware, if you put a lot of grease on the external drive splines where the wheel goes, it will sling it all over the rear wheel. I ride the bike about 200 miles, clean up the rear wheel, and don't have the problem again. I have never replaced an o-ring, but if you are going to, I recommend the stock one. O-rings from home improvement stores are mostly meant for faucets, etc., and may not hold up as well as the Kawasaki part. You don't have to replace the o-ring every time you do a spline lube, so $4 sounds pretty cheap to me. Just keep grease on those things. I see no reason why CV joint grease wouldn't be just fine.
Did you get a maintenance free battery? I highly recommend them for the VN750, for several reasons. One, it is a real pain checking the fluid level, which needs to be done every couple of weeks if you ride very much, and to do it you have to remove both the seat and the battery. Back before I got my first maintenance free battery, I rode with no bolts in my seat (I had an extended backrest to help hold the seat in place) because opening and closing Kawasaki's joke for a tool box door every 2 weeks to get to the seat bolts will soon cause it to break off. Another reason is that regular non maintenance free batteries tend to dump acid all over the frame and rear wheel, causing corrosion and rust. I rigged up a catch bottle zip tied to the frame to stick the drain hose in to prevent that. Yet another reason is if you let the battery fluid level get low, it will cause very high resistance in the battery, and I believe that can have something to do with the stators burning out. Replacing a stator requires pulling the engine on a VN750. A maintenance free battery solves all those problems, and tends to work better to boot.
Not sure what you mean on the jets, unless you mean a #40 pilot jet. That shouldn't be a problem, and is probably what the bike should have come with anyway, if it weren't for the EPA. I rejetted to 135 mains and 40 pilots when I installed my Vance&Hines exhaust, and the bike runs better than it ever has. Sounds better too. Jerry.
I am a motorcyclist, NOT a biker.
1997 Vulcan 750, purchased about a week ago
2006 Sportster 1200 Low
2013 Royal Enfield Bullet 500, converted to carb
2001 Yamaha XT225, heavily modified
2004 Honda Rebel 250
1979 Vespa P200E
2002 Vulcan 750 parts bike
1994 Yamaha XT225 parts bike