Yeah, it's really the failure of two separate systems--a) vacuum petcock, and b) float valve in the carbs. If either one of those things had worked correctly you wouldn't have had the gas in the oil problem that you've got.
Pulling and cleaning the carbs is a major undertaking on this bike as the designers jammed those carbs in there with a shoehorn (after greasing them up first!). That's why it's probably much easier to fix the petcock instead. I've never actually fixed a vacuum petcock myself. Many people will just replace the petcock with an aftermarket style when the diaphragm fails, which is what I have done in the past. The aftermarket petcock will have an "off" position, which serves the same purpose as the vacuum shutoff. The thing is that you'd have to remember to shut off the petcock and turn it back to "on" every time you stop and start your bike as long as the float valve is sticking.
The easy way (may not always be effective) to free up your float valves is to empty out that old gas and then put some fuel additive in there to help clean up the carbs. Lots of people swear by sea foam, but I've never had much luck with it. I prefer the "Berryman's B12" additive, myself. The best remedy is to run a few tankfuls of gas through the carbs for everything to get cleaned out like it should be!
Then you'll always want to use a fuel treatment like Stabil (or even sea foam is probably fine for this) whenever you store the bike for extended periods. I always treat my fuel when it's sitting over winter.