Virago 250 misfires in wet weather only

At this website www.reliable-store.com I found so much of information on how to repair vehicles and technical issues in detail.. Also complete, much better
language, and they also have live assistance feature

www.manuals.org ... if they are still around

check side stand warning switch

is the smell coming from the exhaust also check to make sure nothing is leaking out the pipes then get back to me if not

Some models have a 'kill switch' if the sidestand is still out. So the contact, telling the bike that the sidestand is it, might be damaged.

You can cut them and see if that works but you might have to connect them together if it is a closed loop circuit. just use a meter to check the voltage with the stand up and down. if one wire is hot with it up or down you will need to connect the two to trick the bike into thinking the stand is up. But they are both hot then cut the wires and terminate them so they don,t touch anything and you should be good.

I sounds by your explanation, that the belt which drives the alternator is slipping when it gets wet.
When this happens the alternator stops charging the battery, and when battery voltage drops the engine cuts out and won't start due to not having enough current to supply electrons as well as starter motor.
Have the belt tension checked, as well as charging output of alternator, should fix it.

spay all electrical leads and boxes with wd40 to repell water