Hi,
As usually - it depends. Generally if we'd talk only about execution plan, tables DDL and keywords then it is not a big deal. It can be done similarly to how PostgreSQL extension was done. It is tiny, just a few additional functions needed (although execution plan support maybe more complicated).
If we'd talk about fully-functional extension (which supports all non-standard entities, user/sessions management, maybe some non-standard data types, etc) then it is more complicated. Could be similar to MySQL extension.
Unfortunately I myself don't even have access to any live instance of Vertica and can't implement even first type solution.
If you familiar with Vertica internals (even better if you know Java) - please write me on
serge@jkiss.org, we'll figure something out.
Regards