A. The short answer is keep to the stock tire size, because it is both Yamaha's and the tire manufacturer's recommended size for the rim width and fit on the specific model of bike.
Comment: This is a topic that rears it's head on a regular basis. Here are a few friendly reminders to keep in mind when dealing with the urge to put fat tires on your Virago. The only different in this information for the earlier years is the stock tire size on the '81 to '83 Viragos. They came stock from Yamaha with a 130/90x16 tire size, while the stock tire on the '84 up Viragos was 140/90x15.
The rim is a fixed width, and motorcycle rims have a different bead seal shape than cars. So if you put too wide a tire on your rim, even if it clears the swing arm/chain/frame, you may simply create a situation where you loose the seal at the bead in a turn, and the bike will go down. Sudden rapid deflation from a too wide tire has killed a few bikers. It has injured MANY bikers. (when you loose the bead seal with a tube in the tire, the tube is often destroyed instantly when it gets pinched between rim and tire) 
When you ride, your tire sidewalls flex constantly. Be sure to leave ample clearances for this! In a tight curve, a tire's flex can easily use up between 1/4" to 1/2 " of clearances. Just be aware of that. 
Also, I have a 1985 XV700 and here's what I found - tire manufactures widths differ considerably. Period. I can run a Metzler or a Cheng Shin (now Maxxis) tire that will exactly match the OEM 140/90 that came on the rear. The same tire size in a Dunlop or Avon is significantly wider, and hits the shaft tube before it's even mounted all the way. The Maxxis and Cheng Shin tires allow me to run a 150/90 with no problems, and leave me with 1/4" to 5/16 inch clearance to the stock shaft tube. 
So far, I've learned that the 150/90s and 150/80s are the only wider sizes that fit correctly on the stock rim width to stay on in a tight turn or emergency maneuver. You don't even want to know how I found this out. (But if you don't trust my experience, buy a lot of disinfectant & some bulk gauze and medical tape before you test my findings with anything wider on your own. Road Rash on arms, legs and hips hurts like hell, and the skin heals scarred.) 
If you want to see if you can fit a specific manufacturer's 160/80 or 160/90, measure it ON the rim, aired up. The width and height of a tire changes from what it measures without a rim or air in it. Also, remember at between 40 and 70mph, the tire height "grows" quite a bit from the forces of spinning, and the width shrinks a bit (but not much). 
If you still decide to take all of the tolerances out of the clearance area, be aware of several other interesting things. You CAN shrink the shaft tube to within just millimeters away from the shaft with just cutting a >----< in the tube, crushing it (carefully!) and then welding it back together again. Be sure to know what the clearances between the shaft tube wall and the shaft are before you start, and take the same measurements again afterward to be sure no side-pressure will be put on the shaft from the new welded area. (You don't want to know what it'll cost to find/replace the U joint on the output for the shaft area!)
Additional Information: Another thing to consider is that no manufactures will  warrantee a tire that has had unusual wear on the sidewall from too tight clearances. It is considered "intentional abuse" and is not covered under any tire warrantees. You eat 100% of the replacement cost. All tire manufacturers will also tell you, that they have no legal responsibility to you at all, if you put a larger than recommended tire on your bike. You're on your own, literally. 
Lastly, your insurance company WILL NOT PAY on your policy if you make a claim for an accident that is later determined to have been caused by your putting too wide a tire on a too small rim.

My thanks go to "Hahnda" for providing a link to the following article on motorcycle tires in the forum at Chopper Underground. The article, entitled "The Mystical art of Tire Reading or It's black, it's round, and it goes on the wheel" by Josh Fielek  can give you a little better understanding of your motorcycle's tires, and what tire sizes can fit on what rim.

Here's a couple of handy tire fitment charts that may help you decide what will fit and work safely.