I don't think a veto is worth a damn without discussion. Just to get up one day and say to my husband "Hey, I don't like this new girl. Stop seeing her." rubs me the wrong way.
I trust his judgement enough that if I really have concerns about a metamour we can address them together. Same on my end. This is all after I have done my own mentalwork when it comes to polyamory in general, so trust me I didn't always feel this way, lol.