Dylan implied repeatedly that he knew some bad stuff about Stuart, but did not specify what. By the time he *finally* revealed it to Brandon (during the flight to Vegas), it did not really matter anymore. Why? Brenda was still important to Dylan. Whether or not that information would have made Brenda break up with Stuart, didn't she deserve to know the truth about him before progressing their relationship?
shareHe always get mysterious when he has information about bad guys (Jake, Stuart, Padilla...)
I assume he didn't want to get involved as he didn't want to interfere in Brenda's love life, being her ex and especially after betraying her himself. But he did watch for her talking to Stuart about it.
He didn't want to interfere either when they decided to get married and Jim asked him to talk her out of it. When he did get involved, she got really mad at him.
He knew her and was aware that if anybody tried to talk her out of it or badmouthed Stuart, she would do the exact opposite.
When in Vegas, Dylan actually realizes that Stuart has changed and is not in this business anymore. So he doesn't want to spoil the whole thing, nor does Brandon.
Where's the fun in that??
Stories must unfold. Dylan couldn't reveal all that crap right away....We had to be held in suspense along with the other characters