Well the only real official explanation we've gotten about Vulcan hull forms comes from A Choice of Futures
(Rise of the Federation Vol.1) where Tobin Dax describes that the hull silhouette is in fact a result of the stated design goal of Vulcan ships rather than a result of warp dynamics. Specifically as combat oriented designs they're designed to have small forward aspect ratios to present a smaller target to the enemy.
The lack of maneuverability is also noted as a feature of coleoptric drives. The combination leads me to believe that they're able to project a very smooth, very even warp field. Also the sheer diameter of the "coil" would essentially make a single unified field, something like putting a very wide aspect warp nacelle together.
The end result would basically be a hull form that would try to make up for the width of the nacelle if it wanted better peak transition efficiency.