[Speed Trigger] can be replaced for [Vile Acceleration] but the difference in charge rate between [Speed Trigger] and [Vile Acceleration] is negligible at 0.43 vs. 0.55 when compared to Lenz original charge of 1.2.
Speed Trigger slot could also be flexed for a number of other mods:
Heavy Caliber - Increase damage to 15K +
Argon scope - Increase Crit Chance
Vigilante Armaments* - Increase Mutlishot to 2.5
Notably - I have set up the Forma investment to achieve the build and allow flex on slot 7 as the original had a 7 Forma build.