Could be that the variation in pitch on lower notes gives the resulting pitch go lower then C3 in the low peak of vibrato amplitude. LSDJ can't go lower then that which might cut these parts away. Only theory.

Title says all really.
Every time I use a high vibrato on instruments they seem to be less loud. This is especially noticeable with notes C3-B4 in the pulse channels.
I don't really understand why.
Can someone explain this?

I usually keep pulse instruments under A8 env. So I don't want to set these instruments on C env or louder. But sometimes they reallly drown so I have to pump them up.
Is it just a mastering/mixing thing and shouldn't I put them louder?