Because the basic idea of vigorous pace is subjective, I would suggest weighing how you feel at the end of your set. If you find yourself out of breath and spent, then it is probably safe to call it "vigorous", whereas if you feel like you could another 100 lengths, you might want to consider it leisurely.

I split my time, using leisurely/moderate for warm-ups and rigorous for sprinting and kick drills. It's a subjective measure. For a novice swimmer, all swimming would all be vigorous. For an elite swimmer, the same set at the same pace could be leisurely.

Calorie-wise, this seems to be working for me.

It really depends on your fitness level. It could be leisurely for some people, but there are many people that couldn't even do that much in an hour if they were going at 100%. I'd see how tired you were, did it feel vigorous? Did you feel totally spent afterwards, or could you keep going? I've been logging all of my "main sets" as vigorous for the time that I'm actually swimming. For example, if I do a set of 8X 75's on 1:20, but get 20 seconds rest on each one, then I count that as 8 minutes of vigorous swimming, since I was swimming really hard the whole time. I don't count rest time since I'm not swimming. For the rest of my set, warmup, cool down, drills, or distance work where I'm not swimming too hard, I just count the amount of time and log it as moderate. Sometimes I'll be going vigorously, but sometimes I'll be resting at the wall while changing to a new activity. I think it'll all work out in the end. I also aim to eat back about half of my exercise calories, so the actual number of calories burned isn't too important for me.

Thanks that's great.