Hi Aminator,
there are surprisingly only a few tanlines decals out there, given the fact that those are comparably easy to create. Maybe there are some more of them hidden in some look-files, I don't know. It seems, that the users are somewhat happy with what they have. VaM is a great playground for creative people. It is relatively simple to create your own stuff or editing stuff from other creators, if you have your own ideas or wishes.
If you have basic knowledge of Photoshop, I strongly suggest to try it for yourself.
The tanlines textures are comparably simple (to other body areas):
The problematic areas are only the seams.
With those tanlines this areas are the back seam, and the seam at the inside of the legs.
If you stay away from those seams, almost nothing can go wrong.
Especially the sides of the hips are somewhat unproblematic, and this is where your "pre '80th" tanlines would maybe have the biggest difference to modern swimwear.
It is only a matter of some try and error to find the right curves.
-I would suggest to use the one piece bathing suit tanlines in one Photoshop layer,
-the bikini tanlines in an other layer,
-and a body texture of your choice in the basic layer.
-Now compare the two tanlines while blending it over the body texture.
-Now delete any areas you don't need from the one piece bathing suit,
-or add areas from the one piece to the bikini bathing suit.
-Mix and match till you are somewhat content.
-Hide the other layers and save your own tanlines as png file, like the originals.
-If it doesn't work with only hiding the background layer (body texture), then rightclick on it and change the "background to layer".
-If you have the desktop version of VaM in the background, you can swap back and forth to Photoshop, to see the changes.
I guess, this will last maybe as long as I have needed to write this.

If you want to share your edited tanlines, make sure to give credit to the original creators.