I have not flown Viva Aerobus ex-LAX but in Guadalajara they are very strict. They have portable scales and will randomly pick passengers out of the boarding line at the gate to weigh their carryon bag or personal item. I would check your bag if there is any doubt about it meeting the rules to be a carryon. I have never checked a bag with Viva Aerobus but I think the ULCCs can do a pretty good job with checked bags since they tend to have relatively few checked bags per flight. E.g. on a non-full ULCC flight I was on a few weeks ago, there was a grand total.of 35 checked bags.
Viva Aerobus is trustworthy and will honor the extras you purchase. If not, complain like crazy to the DoT and PROFECO. There is also a big enforcement action in Mexico right now against the ULCCs for not including two carryon bags with all fares (as is supposedly required by the Mexican Civil Aviation law). So Viva Aerobus might not be as aggressive about charging for carryon bags right now. A backpack also gives you some flexibility since it can be crammed in to a sizer as long as it meets the weight limit (or you can take clothes out of the bag and wear them to get under the limit).