Unless the seam is hidden in an area such as being centered in the grout line of a tile patterned vinyl (and even then) it will be visible; what you want to control is just HOW visible the seam will be.
This is best controlled by assuring a good pattern match, choosing a good place in the pattern for the seam when possible, and getting a precise mating between the two pieces by "double-cutting."
Use a utility knife with a brand new blade and a clean metal straightedge as a guide. Overlap the two pieces, being sure the pattern aligns on both. Cut through both pieces simultaneously in one pass, being careful to hold the knife steady & vertical as you cut. As you are joining to vinyl already installed, and in a doorway, you only want to remove a narrow strip from the edge of the existing floor.1/4 to 1/8th inch is good; trying to remove too little doesn't work very well.
Lift the top piece to remove the scrap edge cut off of the bottom piece being careful not to handle the actual seam edge. Then, after making sure there is no excess adhesive or debris in the area, press the sheets down into your adhesive. The joint should be slightly "full", but not puckering up, and can be carefully rolled with your roller parallel to the seam or massaged with a soft, clean cloth, again, working down the seam & not across it.
Finish by sealing the seam with the proper seam seal kit for use with that particular brand flooring. Some kits work for several brands, while others are very product specific.
Keep traffic & moisture off the area while the seam dries; 24 hrs is desireable when possible.