[Coodeville]

I've been sewing a lot of vinyl onto sewfoam and practicing straight lines. So far, so good. The other day I decided to do some cleaning in my home and made a small wallet to store some credit cards, business cards, etc. It did not have to come out perfect at all. So I sewed two pieces of vinyl together and my bobbin thread was pretty much straight along the bottom - of which I expected. So the question is this _ does my thread tension need to be tightened or loosened when going from a thick piece of work to a thin piece? I have not yet experimented with this because everything I've done so far has been very thick.