So, was doing some looking around the forum, and maybe I missed something, but the only thread I found was this.

So, Druid player. Aasimar Druid. Has a Tiger as an Animal Companion, and found the Vassal of Bahamut PrC. Her Tiger recently gained his 15th HD, and as per the Druid rules (and MM Vol 1 rules, and Savage Species rules) her tiger is due another Feat. And can take and gain the benefits of the Vow Feats. No issues there.

She wants Lazaroth (tiger) to take Sacred Vow.
My argument is you you can't take a Vow if you're not Sentient. (Tigers have 2 INT, sentience is 3+ in 3.5e)
She countered "If I take Exalted Companion, he gets 3 INT", and she's right.

Thing is, she knows I know she wants to have Laz take the Vassal Of Bahamut PrC. Which, RAW, a sentient creature could do if they meet all prerequisites.

My question is: What do?

Should I allow this, and if not, why? And how would I go about saying no without sounding like an ass "I'm the DM, I said no. I don't need a reason"