It isn't possible to mix any generation cloth bases, and has nothing to do with the file ending.
G1 clothing works only with G1, G2 clothing only with G2, etc., etc.
Do you know which generation of clothing that you own? The last .vns cloth that was available was for G2 - after that everything changed to iUpper/iLower etc.
System: Win 10 Pro (1909), Asus X99-E WS, CPU i7-5930K -3,5 GHz, 32 GB DDR4 2666-16 RAM, NVidia GTX 1080 Ti GPU - 11 GB VRAM(Driver Studio-442.19), Samsung 850 Pro 512 GB SSD, 6 TB HD storage.