Vinyl flooring question

I purchased a house about 1.5 years ago and the kitchen and dining room share the same style vinyl flooring. My problem is the flooring has a seam between the two rooms that the builder never sealed properly. He simply put a piece of masking tape over the seam and said he would fix it after we closed on the house.
Needless to say, he never fixed it (among other things). I'm already in a legal fight with him over a lot of things done incorrectly so getting him to fix it is not a solution.
How should this seam be done properly? The seam is approximately 10 feet in length and tape has long since come up. The vinyl has started to curl very slightly at this point and I'd like to get it fixed before it has to be replaced.

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