Zumi will fite you for the statement Vivillon >Crobat lol Anyhow, would you be willing to breed for new mons tho? If it is the case, i would breed a new Swinub if i were in your shoes, Mamoswine is very good only if you breed Icicle Crash into it, Blizzard being special move(only for removing field, given you already defeated Amber rendered Blizzard useless) does not suit him well as its special attack is pitiful compared to its massive physical stats(130 base attack is much impressive) and keep the priority move Ice Shard(priority is important in case of slow mons even if the base power isn't great), also slap an everstone to one of the parents with Jolly nature in daycare, the off spring will inherit the nature, Jolly nature is best nature for Mamoswine in game(Adamant is next best if you don't have jolly). 80 base speed for Mamoswine isn't bad in contrary to Tyrantrum's 70(but then again Tyrantrum has Dragon Dance breedable to it, somehow amending its poor speed). In any case, i would advocate breeding Tyrantrum and Mamoswine for a better nature and movepool, and i don't see any reason why you shouldn't keep both of them in your party, perhaps for reserve team or rotation? After all, we still have a lot of gyms to go, both of them have different counter and checks for those gyms.
Follow up: Yanmega > Vivillon with same typing in my opinion, Speed Boost Yanmega can wreck houses when given chance to set up with Protect/Substitute.