If taking the extremely high risk of approaching random shady looking vans wear a body cam. They sell them online. Or at least have your cell phone recording video and put it in a shirt pocket to act like a body cam. Live stream if possible in case they take or smash your phone. Others should also be recording you at a distance in case shady person drops you. Give the footage including license plates to the local authorities. I personally would also mention to dispatch that due to the dark windows you could not tell if they have a firearm. Other potential advice would depend on the laws of where you reside. I am not a lawyer just an internet armchair quarterback. Oh and that van is probably stolen, he is probably on drugs and in his mind you are probably in his turf. If you have a network of neighbors you trust be sure to also let them know of the existing risk.
Otherwise take rawgabbit's advice and stay clear of the drama. Just get video and pictures without a flash.
To answer your question some dodgy private investigators find questionable methods of looking up plates and will happily charge you way too much money to look up that stolen van.
but in the cheaper, but legally grey area, you can usually lookup a plate for a few dollars if you have a "legitimate reason" such as the creep damaged your property or theft. that's a he-said-she-said thing, so ...
https://www.dmv.ca.gov/portal/file/request-for-record-inform...
of course this is USA, California. Other US states are mostly similar.
there are 100s of online sites that are "private detectives" that will run a plate for a slightly larger fee. this may be faster and less grey than diy dmv requests.
if you are a real karen/ken you can post a pic of the van and plate to nextdoor, etc. and make a big post about the creepy van.
It doesn't mean I want to but I was placed in the position of responsibility to love and protect my spouse and children, not to rollover and die at the first sign of trouble.
Plenty of people are bigmouths and quick to shut up when they realize they might get what they asked for.
Whether armed or unarmed, your best bet is calling the cops: that was my point. They are experienced in establishing whether there was intent of real harm, and providing unbiased evidence of illegal behaviour like threats to courts.
In most of the world, you using a weapon at the first "sign of trouble" would be considered an excessive response, and for good reason — which means that you might end up in more legal trouble than the person threatening you.