One would not mix victoza with sitagliptin because they both act to increase GLP-1 levels; sitagliptin inhibts the breakdown of GLP-1 and victoza is human GLP-1 with a side chain added to make it last.
Main side effect of victoza as folk say is nausea - this can be reduced by taking on an empty stomach; if nausea is still a problem, then one can reduce the dose and increase very gently by one click per week.
In the LEADER trial, victoza reduced rate of heart attack, stroke and death - quite handy.
It decreases weight even in folk who aren't really sick on it - folk just eat less cos it switches off appetite.
It is probably the best once per day member of its class given the benefit on vascular events.
I think you should ask your GP if you are meant to continue the victoza plus sitagliptin. If you need a different tablet to help the victoza, there is quite a choice now.
Best wishes