A couple thoughts...
first, I have a dear, dear friend who had RNY way back before we all knew about the DS. She and I and another friend were at a hotel together, and she recommended that anyone who wanted to use the bathroom should go before she did, as no one would want to go in there for awhile after her. Very considerate, but tells you that at least some people have issues with smells even with the minimal amount of intestine that is bypassed with RNY. There is also a guy with SIPS who posts a lot on that other website (we don't mention the name of the website here but you know what I mean) and raised concerns about a long airplane trip, using airplane restroom, wearing charcoal lined undies, etc. And that's with SIPS! I have no idea what he eats, and would never want to confront him or offend him as he seems like a nice person and that's not my style, but if anyone thinks that having SIPS or RNY instead of DS is some kind of guarantee against inconvenient poop or smells, they are wrong. It's far more dependent on your diet, and to some extent individual differences.
Regarding vegan diet - I'm very glad you're open to eating at least some animal derived stuff, and there is at least one person here who does mostly vegan but with eggs and fish and does well. I'm not sure if she consumes any dairy. Many people with DS or RNY, and presumably SIPS, become lactose intolerant, but most of us can manage cheese, just not milk itself, so cheese is a good source of protein for us. Remember, you don't have to worry about the fat or calories anymore with DS. I would recommend that you be open to other kinds of fish and shellfish and not just salmon. I think it would be very tough to get in enough protein with being strictly vegan. Please understand that I greatly admire people who choose to be vegan, this is NOT a criticism, but we do malabsorb about 40 - 60% or the protein we eat, so between that and some degree of restriction with your sleeve, you need some concentrated sources or protein. Also, you need a variety of different proteins to make sure you get in all the essential amino acids.