Dave,
My foc went from 10.5% to 12%. My arrows are 27 in. long.
The Blazers weigh5 grains per. and the AAE Plastifletch 4'' vanes I was using were ( I Think) 8.5 grains. 5 in. Bohning vanes are pretty heavy at 12 grains so your foc will go up more. Maybe 3+ %.

It makes a little difference and the small amount of forward weight will help stabilize your arrow.