I would go with vroom, not va-va voom, because of the dictionary definition.
in British English
the quality of being interesting, exciting, or sexually appealing
Collins English Dictionary. Copyright © HarperCollins Publishers
Another reason why: we say an engine “purrs”. A purr, by definition, is a vibrating noise or hum. “Vroom” definitely is a vibration, but not va-va voom.
When the engine of a machine such as a car purrs, it is working and making a quiet, continuous, vibrating sound.
Both boats purred out of the cave mouth and into open water. [VERB preposition]
The sleek car purred down the country road. [VERB preposition]
Purr is also a noun.
Carmela heard the purr of a motor-cycle coming up the drive. [+ of]
Hope that helps.