Depends on the type of menthol you're looking for, a tobacco / menthol, or just straight up menthol.
One of my favorites is simply unflavored nicotine juice with some menthol crystals or menthol flavor
added to it. Just make sure the flavoring is nothing but menthol crystals and PGA mix. You just add
one or tow drops at a time until it get to the strength you like, count the drops and you can reproduce
the results easily.