having vvvv perform sql queries is on a very old wish list.
for now the workaround would be having a php/python/perl/whatever script which exposes the sql acess to the database over a tcp/ip socket. with that you would be able to exchange strings using the TCP object between vvvv and your data base. so you could send SQL statements and get the results as a return.
Another option is using HTTP Get. If you have a script on a web server which does a certain query and returns the results as a web page, you can easily get the xml with the HTTP (Get) node. you can easily try this with any web based database which puts its query strings in the URL.
For a future version, we´ll promise a HTTP (Post) node - this would allow you to edit the database by posting data to the forms of something like phpMyAdmin.
Anyway, for now you could even try to build that yourself with the TCP node.