No, this is fairly reasonable
By using the spell as a bonus action, this is going to cause your bard to lose out on several things:
- The ability to cast larger spells
- Bonus Actions for other uses (Bardic Inspiration, Healing Word)
Since you mentioned that he isn't planning to deal damage, he likely isn't going to be using his main action for an attacking cantrip or weapon attack. This leaves his main action to be used for skills, defensive spells, or Dodge, which are all pretty low effect in combat.
Compare this to the Mastermind, who can use the Help action at 30 foot range, who can:
- Grant an ally guaranteed advantage every turn on their choice of action (Vicious Mockery requires a failed saving throw to do anything, and only impacts an enemy's attack)
- Attack with the main action
- Land sneak attacks with his main action (his primary class feature).
If you do this, realize that this bard is going to be losing out on a lot of utility he could normally use his bonus action for, like the best level 1 healing spell, Healing Word, and his class feature, Bardic Inspiration. Without this change, he would be able to cast VM and still use those two resources, but now that he cannot, he will be fairly limited in the low levels.
This change would have more value for someone who was aggressive (using spell slots to hurt enemies, uses main action to attack), but for a pacifist, this actually doesn't mechanically make him much better than any other bard.