There have been many many folks here that have added the additional pully without any problems. There's a lot of give to the tensioner pulley, so there's no reason to change the belt length.
The belt has a long distance between the pulleys, and the new pulley will shorten that, and take out the "flutter" that's making the vibration. It'll probably take you longer to get your tools out than install the pulley.
Every day's a learning experience, though sometimes not what we expected.
2012 Jay Flight 19RD
2016 Ford F150 XLT 2X4 SC 3.5L Eco Max Tow
2010 Tundra TRD DBL Cab (Traded)
2 new fluffy Corgis, Bayley and Stanley