The reed valve ports are on the heads, front on the right, rear on the left. They are where the BIG hoses go to from the air injection valve under the seat. You can get "coasters" to do a really neat clean job, I just went down to Pep Boys, in the HELP! section, and picked a couple of the rubber caps made to plug off car heater hose connections. I don't remember the size, but there are 2 sizes, and these were the smaller ones.
I would remove the tank, pour out all the gas, pour in about a gallon of fresh gas, close the cap, shake the hell out of it for a couple of minutes, then pour out that gas. Next, remove, disassemble, and clean the carbs. They are probably filthy inside. It is not an easy or fun job on a Vulcan 750, but it is the only way to get them clean. Once that is done, put everything back, pour 4 oz of Seafoam in the tank, and fill it up with gas. If the carbs are clean, and gas is getting to them, it should fire right up. Here, a 16 oz can of Seafoam costs $10, but that is enough to treat 4 tankfuls of gas on the Vulcan. By then, everything should be clean. Use a can about every 3 months. I drain the gas out of my carbs about every 3 months, remove the fuel lines from the tank, and use them to fill the carbs full of Seafoam. I let it soak for 24 hours, drain it out, hook the fuel lines back up, and fill the carbs back up with gas. I then use the rest of the can of Seafoam in the next 3 tanks of gas. That pretty well keeps the fuel system clean. Jerry.