It depends. Beating a bad out-of-conference team is worse, because it proportionally weakens you in the strength-of-schedule tie-breaker in playoff determinations (since you've made your opponents' overall record worse.) Beating a good out-of-conference team has an impact on your strength of schedule, of course, but it's a proportionally lesser impact.
So really, the degree to which the Bills damaged their playoff hopes today depends ultimately on how good the Vikings' record is. If they end 9 and 7, then the Bills' win is only a 10% reduction in the Vikings' contribution to strength-of-schedule (because if the Bills lost, the Vikes would be 10-6, so that one game is 10% of their total.) But if the Vikings end up 4-12, then it's a 20% reduction, which is a significant impact to the Bills' playoff tie-breaking chances.