less-than-unity inverting op amp oscillating. Why?
Check the data sheet on the op-amp. Above about 1 MHz, the closed loop output impedance is about 400 ohms and if there your VCO uses a varicap diode then bingo, that's where the unwanted/unexpected signal is coming from. Try feeding it via a resistor and capacitor with the cap closest the VCO control port.

I'm aware you say that the oscillator should be 10MHz but could it be slipping up to 11MHz and you are seeing an overtone on the vco input?

Here's the section in the data sheet: -



Maybe try a different op-amp.