Personally, I would do nothing to the ants, I'd expand the vegetable patch, and.... Let them have the area they've claimed.
I garden with fire ants... Mostly not a problem.
http://stonethegardener.tumblr...
In droughts, they will raid the tomatoes for moisture, but mostly they are beneficial.
What we actually need...
Is more information... What kind of bed are you gardening in, how many years?
What's the weather like, what amendments are you using?
In all honesty, your description of the problem seems very unusual, I think there is something else going on.
