Len, a couple of thoughts; have you called home warranty to find out the original roof installer and the make and MFG of the vents? Most of the vents I have seen have flanges under the shingles, so replacing the vents may require removing or at least lifting some shingles to get the old vent off and the new vent on. My other concern would be the age of the roof, if the shingles are old they may have become brittle. With the info on the vents it may be possible to determine if these can be repaired without impacting the roof.
I have no specific knowledge of the vents on these houses, back up North when we had the roof replaced, I saw what was done, and all of the penetrations had flanges, and I think some shingles were installed, then the vent, then the rest of the shingles so the vent flange was actually between 2 layers of shingles.
Hope this helps.
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Pennsylvania, for 60+ years, most recently, Allentown, now TV. 
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