1. My first preference is vBulletin as I've used that forum software in the past. Question: is any bridge out there to make this integration flawless?
Every forum platform is good. Depends what you want and what you search.
2. Is there any other bridge to integrate well with Elxis I mean any other capable forum software?
No, also because your security is a risk. Is better use a component native for Elxis than not native, or those extensions (components, etc) modified and after checked to be used under Elxis.
3. I see you use here SMF...is this good compared with vBulletin or phpBB? My community can grou up to couple of 1000s of users...
3. We use SMF because eForum is a new component, and times ago eForum don't exist, otherwise we used eForum
4. From your experience would be better to place my forum in a subdomain or alongside Elxis? If Elxis crashes will Forum crashes as well if placed under the same umbrella? I see you here at Elxis use the subdomain but I would like to know why? Is this just a personal preference?
If u want use a forum in the same domine and integrate with elxis, use eForum, so you'll have many advanges that Elxis offers...in first time security and stability.
In any case you can use the forum platform more like you, but in a subdomain and his database separated from elxis database, so if forum crash, only the forum crash
and the Elxis security is not a risk