This is a known issue related to a change in Vertica's naming convention for their drivers. I can't comment on an ETA for supporting Vertica 5.1, but I encourage you to contact support to explain the impact this has on your business so they may try to escalate the effort.
Here is our position from Tableau about connecting to Vertica 5.1 (as of Feb 14, 2012):
HP released a new version of Vertica, version 5.1, in early 2012. The ODBC driver released in conjunction with this Vertica release has changed, which causes Tableau to not be able to connect. Tableau is working on changes to our products to enable successful connections to this new Vertica 5.1 version and is targeting delivery as part of a maintenance release of the Tableau products in late March.