You need to port the existing users. If you want to be conservative, then port those active in the past XXX days. Arguably not a serious concern, but the simple fact is that folks are tied to their identity, and if the users are not ported, then their identity is up for grabs for anyone to take.
If you can port the moot users, you can port all the users. No reason to port passwords, just port the contact email and let them run the "forgot password" service to log back in.
You really don't want to keep the old site alive.
The frank blunt truth is that there's marginal bandwidth to keep this site alive as is, adding another, even a read only site, is still going to be a great sucking sound of time and energy as kernels, tools, userspace, etc. evolve.
Keeping ancient, stagnant software is actually a lot of work. Keeping the "old site" is basically a death warrant, just a matter of when.
Porting the messages to the new site eliminates that problem, it gets maintained with all the rest. VB is ubiquitous enough that it should be doable. I imagine there's a lot of expertise in this user base that could help. I could help, PHP isn't my thing, but I have a true knack for this stuff. SQL is bread and butter stuff. I've been keeping arcane computer things alive for 30 years, and "this is Unix...I know this."
There's, what, 15+ years of content on this site. There's the common saying that "the internet never forgets", but it's not true. The Internet is a combination of a patient with Alzheimer's, and their mother. Forgetting most of the details, but remembering only the embarrassing parts.