In gambling terminology, "bear" and "bull" refer respectively to thinking that a bet is bad or good, so a "tesla bear" would be someone who thinks that "tesla will be more valuable in the future" is a bad bet
I think they were referring to the Bear/Bull comparisons made to the NCR and the Legion since those are their respective symbols. Plus in lonesome road Ulysses is constantly going on about the Bear and the Bull when talking about them
There was surprisingly low amount of gambling there for something with "Vegas" in name.
Poland fact: in Polish, the word for gambling is literally "hazard" which would tell you everything you need to know (but it means nothing more than gambling in Polish).
i assume it gets into gambling from stonks bear and bull markets but stock markets and gambling are kinda the same so it's possible it's older in explicit games of chance