It is extremely difficult to design a good VCO. But you can take the fancy DSP out of the equation if you “borrow” the VCO code from the VCV Fundamental modules. it’s still not going to be super easy if you are not a good programmer.
I have this demo repo that talks a little bit about making VCOs. starts with a terrible one, and works up to one that’s like VCO Fundamental was, sort of, back then.
Get stuck in, and don’t worry too much about aliasing and other things so essential to modern DSP. If you have a good algorithm which sounds nice, sure it can be made better by oversampling and knowing a lot about windowing and sinc(x) along with things which are just faster but do the same almost. You could spend many hours and still never reach peak DSP.