My first thought is that your Zenith coils should be able to handle a 70vg/30pg
e-liquid. Try it and see if you get a burnt taste or dry hits. Make sure that you prime the coils properly, first. Not just a few drops of e-liquid, make sure the cotton is thoroughly saturated.
And "break in" your coils gradually like this user does with his coils.
The main issue with MTL coils is the feed holes are generally on the small side. Vg is thicker in consistancy than Pg, so it doesn't wick as fast as Pg. So the e-liquid may have a tough time reaching the cotton wick. That can lead to a burnt cotton taste or a dry hit with really thick e-liquids.
By comparison, subohm DTL coils have much larger or more feed holes to allow the user to use 100% Vg.