Dear Jimmie,
First, one can not stretch vinyl flooring, as fragile as that stuff is, it will rip. I ran into this problem some time ago. The fella did his own installation and used a water based "contact cement". The chemical reaction ensuing actually shrunk the vinyl. Replace the floor and use resilient tile. ( See other posts for my opinion of vinyl flooring.) Get the installer back there and force him to reveal what adhesive he used. Then demand a full refund.
Len

Hi jimmieb,
Iceman is right.Some years ago,I got into a pinch to re-rent an apartment in a short turnaround time.I installed new vinyl baseboard with an incompatible adhesive.All the seams separated as the vinyl shrank.I learned my lesson!

Hi Jimmieb!

Would you take a look at this picture to see what had happen to this floor, did that shrink? Is it possible to shrunk such a big gap?

Thanks a bunch