You know in the episode in Ben 10 Alien Force where villgax got the omnitrix how could get the omnitrix on him if the omnitrix only goes on the people the has the dna of the tennyson family?

Vilgax knows A LOT about the omintrix.. He's been after it for years and in the original series he wanted to take it bad and seemed like he would use it when he got it, so he probably knew a way to get around the dna requirement. He even knew how to change ben back from alien form WAAAY before ben knew how to do it himself.

what does that mean

the omnitrix evolved and as such the gene code of the omnitrix was changed
and the master code is known by not only ben but by vilgax as he calmed be a master of the omnitrix