It's expensive, but getting at a minimum a new rear wheel in a 5-5/12" width, so you can mount radial tires f & r, will take-care of many of the concerns that enthusiastic VMax riders have, about handling. Usually, that will be a 17" wheel, because your rubber choices are better/more varied, but an 18" rear wheel will come closer to your stock 90-series 15" rear wheel/tire height. The smaller diameter 17" wheel/tire has the effect of gearing the bike lower, so more acceleration, as-if it needs-it. The 17" rear tire/ wheel combo will also cut-down on your top-end a bit, but who's riding around at 145 mph, not-anyone who wants to maintain their riding privileges. You should also run a same-manufacturer and same-style front radial tire, though that may limit your choices a bit as the OEM/stock VMax is a narrow, 18" rim, and proper rim width for the size tire that a VMax would take is 3-1/2". It is not a good idea to mix-and-match different manufacturers' tires, because of differences in construction, even the same manufacturer's tires but different rubber compounds, because you can end-up with an evil-handling bike.
Actually, mounting a good-quality set of premium tires in bias-ply will come-close to the handling of a pair of premium radial tires, for the first 1,000 miles, and at legal speeds, but the handling noticeably deteriorates after that, (concerning miles on the tires, and speeds attained) for me. Radial tires are much-more precise in their handling, should you find yourself at 'big-traffic fine' speeds. It's a shame that Yamaha never made their VMax first-generation wheels in a wider style, because that would have largely-ended concerns with the VMax handling (taking into account its 1980's frame geometry, tire design, and materials). The VMax is no Kawasaki Mach III (ask your motorcycle-riding father or grandfather) in (bad) handling, but it definitely benefits from modern tire technology. It's just expensive to get there. No there are no Yamaha wheels which will swap-in at the rear to give you a 17" or an 18" rear wheel, with proper centering in the swingarm. Also, a 6" width wheel requires a widened swingarm, not hard to-do, if you have the proper tools and the skills.
There are many-more parts of this discussion, but that would be an entire youtube video itself.
Another change to-make, which will greatly-ease your riding issues, is to go to a larger headlight, and modern bulb technology. You don't realize how-much better you can-see at-night until you try a better headlight/lighting. I think that would probably be the #1 thing I would consider doing if I just bought a new-to-me, used Gen I VMax.