I have an 18mm wood floor and am putting vinyl in my bathroom what can I use as a transition strip in the doorway

You will need what they call a reducer which you can ask for at your local building supplier. Tell them the height difference and they will show you which one you need for it. You can go a variety of routes from laminate transition strips to solid wood or metal. I think it's prettier to use a wood or wood-product if you're going from wood to anything else.

I agree and suggest you go back to where you bought the wood and look for their brand of transition strip.