at band practice we dont have a mic amp so we normLLY just put it into the 2nd slot on the bass amp, ive been doin this for about 8 months and my amp i starting to be a bit quiter. does this ruin my amp or am imagining it getting quieter!
Yes, its bad.

EDIT: unless you want to ruin your precious bass amp, then yes it might be good.
It would sound better if you had an amp just for the vocals, but using a bass amp for vocals should be fine.
Is the "second slot" a second input, or a CD In? If the former, it's fine. But try not to plug an instrument into a CD In, as it fucks the amp up a bit.

Additionally, you might have just turned the volume down lower than it was before...
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