I know a guy that has a almost new set of Michelin Harmony 205 55 16 tires for a great deal but I don't see this tire listed on tire rack as fitting my 2007 base vibe.
I know the tire size is correct, is there anything else that would make them not work?
I think Harmony was one of the models phased out when they put out Defender. Tirerack probably just doesn't have access to any more 205/55R16s. The only other thing to make them incompatible would be the speed rating.. Vibes came with H.
They will fit fine - just the speed rating is likely lower, like S or T. Some shops won't install different speed ratings from what the car was specified for when new.
Higher speed rated tires generally have stiffer sidewalls for better handling. I use R rated winter tires without issue here. As long as you aren't the type to attempt top speed runs you'll likely do just fine with the Harmony's.
The Defenders are now offered in the H speed rating for that size. I recently bought a set of Michelin Primacy MXV4's for mine, and I am very happy with them. I really don't push them in corners so I can't comment too much on extreme handling - but the ride and noise are a tremendous improvement from the stock RSA's. The ride isn't nearly as harsh, and noise-wise, it's like somebody sneaked into my car one night and layered the floor in Dynamat! I enjoy the car much more now on the highway - which is a good thing, as I'm typically on the road for between 1.5 and 2 hours per day. Worth every penny to me.
I could have got the Defenders for a few $$ more, but looking at the specifications and ratings, I felt the Primacy's were more suited for my use. The Primacy's are slightly lighter (potentially better ride), and had a better temperature rating. Since I use them strictly during the warmer months and almost all at highway speeds, I figured that the temperature rating may have some significance - as it rates how much the tires will degrade when run hot for long periods of time. Better resistance to heat degradation means better overall life to me - as often I seem to have tires die of weathering/age prior to being fully worn out. Time will tell whether my decision was a good one or not.