Hi,
We went to Vero Beach during 2007 and 2009 spring break season,
and stayed in Inn rooms - We called right at the 7 month and had no issues, I would imagine 1BR, 2BR, and 3BR cottages would be unavailable due to the fact there just not that many of them.
We love Vero Beach and the Inn rooms were fine. I would probably not go again in March, weather was good, but a little windy. Also lots of Man O War washed up on the beach.
We've gone back twice in 2011 and 2013 in Aug, June. Much more beach like weather and no jellies!
Good luck and have a great trip where/when you decide!
Cheers
G4L