I've tried several variations of coasters using ceramic tiles.  Its been alot of trial and error.  I have finaly found the combination that works.  I wanted to be able to put a coating on top of coaster to make it easier to wash them.  I tired Mod Podge and it just didn't work like I wanted it to.  My first try with clear acrylic made the vinyl curl on me.  I finally figured out why.  So here is the secret to make this combination work.  1) You need to use outdoor vinyl.   Indoor vinyl will curl guaranteed if you try to put an acrylic over it.  2) Don't use cheap $2 spray acrylic.  I used Engine acrylic.  I bought mine at a place called Farm-n-Fleet.  I would imagine any car part supply store will sell it.  I bought my vinyl from Nick at www.craftvinyl.com.  Nick has been fantastic.  He ships so fast and packages it perfect so it comes just like you would expect it to be.  He has packs of outdoor vinyl in the clearance sections for $7 for 10 sheets of 12x12.  Thats less than $1.00 a sheet.