Actually this is a little bit too much to give up for Pujols, but I don't think it deserves a veto BECAUSE of the legitimate concerns one of the owners might have about Sheffield's health, and as people who are in the cafe league with me know, I am conservative when it comes to what trades I consider passable.
In the case of Pujols for Sosa and Percival, it's very close, but I think it IS worth a veto, although a lot of people are taking Sosa as early as the second round in fantasy drafts, so with it being that close, I probably would refrain from voting to veto myself.
I will say though that the above trade is an example of a classic fantasy sucker play. Find the owner without a closer and trade him a markedly inferior bat plus a middle tier closer for a super stud. I've seen it too many times to count, and while it follows the "Both teams improve" rule, because a guy is better off losing a point in four or five hitting categories not to come in last place in saves, it's still one that ought to be vetoed a lot of the time.
I'd rather see something like Pujols for Sammy and Rivera. That would be a lot more even value wise and it wouldn't so blatantly scream the sucker play I mentioned above.
As for the last guest, I would be curious as to which trades you have made are getting vetoed. Some public leagues will veto just about anything, but a lot of times people have philosophies like mine, and if give me some for instances, I can give you my take on whether a veto is justified, for what it's worth.