Have only just joined, so my answer is perhaps too late now.
Many years in the plastics industry, later years, specialising in vinyls for buses, ambulances etc.
All joints were welded to prevent water ingress, cos if water gets underneath, smells etc apart from other nasties can occur. And you cannot rectify without lifting the vinyl.
Many people will lay vinyl without adhesive - don't do this!
To maximize the life of the vinyl, use an approved adhesive, if contact adhesive, use a waterbased (acrylic) one and not the contact adhesive for sticking laminates and the like. - That type of adhesive attacks the plasticisers [the additives that keep the vinyl flexible]
One other big point to remember - unroll the vinyl for 24 hours to stabilise before using within the area to have the vinyl installed. If space is a premium and this is usually the case, by all means, cut lengths oversize, stand on end in a very loose 'roll'
Why should you do this? PVC is hot when it is rolled at the factory - a memory of this curve developes, unrolling allows the material to stabilise. Otherwise, if you adhere it to the floor without doing this ensures the vinyl fights against the adhesive and air pockets develop.
When welding, use the welding cord to suit the vinyl in question, leave a 1mm gap between the edges (if using 5mm weldcord) and run an Xacto v cutter along the joint. ensure no glue is contaminationg the join - clean out if need be. then you can commence with welding.
(BTW leave the cord to be cold to the touch before trimming flush.)