You can accomplish such a thing with any virtualization platform that lets you assign USB devices directly to the guest OSes. I honestly can't think of a reason you would need multiple wifi network adapters... across multiple guest OSes... but I'm not in your boat. Virtualbox is a quick & simple solution that does allow for this... but under high-load... can choke from time to time. (I suspect this is true for most virtualization platforms) VMWare's ESX server may not have this problem... but I have not yet tested such.
The why can't I create multiple wifi connections via a single wifi adapter is one of the more interesting questions here. The answer is quite a bit complicated. Microsoft likes to keep things simple by only implementing a very simple set of interfaces that makes things generic enough that any application can make network connections without having to re-invent the wheel every time you want to send a packet of data from one machine to the other. So they provide a basic set of functions that let you send/receive data. WIFI is a relatively new thing in the networking world... and Microsoft tries to pigeon-hole that technology into their existing simple framework. So, they simply added an extension to their existing network stack to add support for the authentication... and called it a day. Unfortunately, Microsoft does not have an infrastructure in place to implement "sub-interfaces" for each physical adapter. For standard Ethernet, this isn't a problem, because you can simply add a 2nd ip address to the same 'interface'. But with wifi, the authentication session is tied directly to the interface.
Where this is physically possible to be done (and is done in other platforms like Linux)... Windows just can't handle it. In theory, the manufacturer could implement a framework to add/delete virtual interfaces on-demand... similar to enterprise-ish network adapters... I doubt there is one out there for Wifi.
You also asked about VLANs... WIFI can indeed carry VLAN'd traffic without issues... the problem is, VLANs are yet another thing Microsoft left out of their framework. Most of the time you get lucky, and the driver has some advanced setting you can set to use a specific VLAN tag... but you cannot typically do multiple tags. Enterprise-ish network cards typically have some sort of control panel that may allow you to setup virtual interfaces... but I doubt there is one such card for WIFI.
