It's okay, you gave Jerry his needed excuse to copy paste his massive experience that his inexplicably led to a complete inability to work on many things.
Just because you can work on some things doesn't mean you can work on everything. I really have no doubts I could disassemble and reassemble a VN750 engine. But like I said, there are a lot of things in it that can't be checked, like in a car engine, and a lot of parts that if they are found to be worn beyond spec, would cost a fortune to replace. If anybody plans to rebuild a VN750 engine, I suggest watching ALL of Andrew Roth's videos (he was known as Roach on here. Don't know what ever happened to him. I located his videos on YouTube.
He never checked the crank or con rod bearings in those videos, he just assumed (or maybe hoped would be a better term) that they were ok. Simply taking an engine apart and putting it back together is not rebuilding it. EVERY single thing must be checked and either found to be within tolerances or replaced, and the design of the Vulcan 750 engine makes that extremely cost prohibitive (which is the case with all Japanese motorcycle engines, it's just that the VN750 has about twice as many parts as the average Japanese v-twin. Way back in 1969, the first CB750 was the first motorcycle engine that was cheaper to replace than rebuild. Before that most motorcycle engines were designed to be rebuilt.
First make sure you are getting fuel to the carbs. Open the drain screw on each one and see if there is gas in the float bowls. The VN750 carbs do not have an accelerator pump, so you may not be getting enough liquid gas into the cylinders to wet the plugs if it doesn't turn over for very long.
All Vulcan 750s turn over slowly, it's just the way they are, but they should continue to turn over. And yes, you should be able to get it to fire on only one cylinder. Another quirk the Vulcan 750 has is after sitting for some time with gas left in the carbs, it will sometimes start up on the front cylinder, and it can take some time for the rear cylinder to kick in. I have no idea why it does this. But if you have spark, compression, and a close to correct air/fuel mixture, then the front cylinder should at least fire. The rear coil is not difficult to replace. A car battery, fully charged, has way more than enough power to start a Vulcan 750. It sounds like you may have some electrical issues. The oil light should also come on. I could have sworn on my 2002 the headlight did not come on until the engine started, but on my newly purchased 1997, the headlight, tail light, and all the instrument lights come on with the key in the on position. It still starts up ok.
There are 2 large hoses that go from the petcock to the carbs. These are fuel lines. The Vulcan has a vacuum operated petcock. The small hose that goes from the front carb is just to supply vacuum to the petcock. The small hose on the rear carb is part of the PAIR (emissions system) which has probably been removed, and it should be plugged up. Oh, there is one more hose that connects to the carbs. That is the float bowl vent. The two carbs share a common float bowl (somewhat unusual) and on a stock bike that vent is plugged into a hole in the right air filter housing.
With the plugs out, there will be no compression, so there should be no change in resistance when turning the engine over by hand. It should turn easily all the way around. I wonder is something is binding inside the engine, making it harder than it should be to turn over. That could overload the starter, make it turn slower, and pull more power than it is supposed to from the battery. It would also make the starter motor get really hot, and I would think it should eventually blow a fuse. The starter is not connected to the engine, so it would have nothing to do with how it feels when turning it over by hand. It sounds like, for some reason, a problem is causing your engine to be harder than normal to turn over.