Yeah, if you look at the study that I linked to, you will see that vo2max training can last 4-6 weeks before detraining starts to really kick in, so my guess is that you were out on the outer edge of that period.
Of course, everyone's body reacts differently. Some people have a naturally high v02max that is not impacted as much by training. Those people will benefit more from tempo work and running economy work that vo2max work, and their "detraining" will be relatively minor. In fact, you might even suspect that they might do better if they worked something other then vo2max most of the time since the upside of the vo2max work is capped, but the other limiting factors may not be. But I believe the high ceiling/low adaptation subset is a relatively small one, and that most people have trainable vo2max capacities that fall within the norms of training and detraining (I guess, by definition, that is true).
If I were training for a half marathon or a marathon, the 12 weeks away from vo2max workouts wouldn't concern me - my Daniels-based marathon training plan does just that in fact; but if I were focusing on the 5k, there is no way I would go 12 weeks without any vo2max work, unless those 12 weeks were from about 22 weeks out from my goal race until about 12 weeks out (at which time I would hit a 6 week vo2max block and then a 4 week taper/race modeling block.