I'm trying to encourage myself to participate in the private beta of vi.SE, but I have to be honest and say that one thing that holds me back somewhat is the thought that if the gods of SE don't deem the site worthy enough to go into public beta, and ultimately to a fully-fledged SE site, all the valuable Q&A we are generating might be lost. Is my concern valid? Would they still be accessible? What has happened to similar private betas?
The posts are archived in a zip file that you can download. The format is not directly readable (you need to program a bit to transform it into something a human can read), but not very complex either. There are a few existing applications to work with these dumps.
Before closing a site, questions that would be on-topic on other sites can be migrated. This only happens if the questions have high-quality answers (which by and large is fine here) and do not just duplicate what is already present elsewhere. If this site doesn't make it, it'll be because almost all the questions are already well-received elsewhere, so most questions should have a place to go. For example, when Embedded Systems and Operating Systems shut down (both due to lack of traffic), a lot of their questions were migrated to overlapping sites such as Stack Overflow, Electrical Engineering, Computer Science, Unix & Linux, etc.
If this site shuts down, I expect that most questions can be migrated to Super User, Unix & Linux, Ask Different or Stack Overflow. This isn't to say that all the questions would be migrated: only the ones that would not be duplicates, or that have better answers than their duplicates elsewhere. The way this works is by collecting migration candidates on a meta thread once the closure of the site has been announced.
I think that the computer graphics beta migrated a few questions out to other sites, which could be an option if there are some really good questions that don't already exist elsewhere.
But generally they are available only via the data dumps.