Could I directly connect CML buffer to the output of LC VCO tank?
If the CML buffer is too strong, it might couple back to the VCO tank.

The alternative is to use ac couple to CML buffer. however it reduces the output amplitude and the tuning range.

What is the common practice?

ac couple might introduce additional loss to the tank if the R after the ac cap is too low....

Remember that ac coupling cap has parasitics to ground so you're amplitude will become lower before it hits your buffer.

Also the ac coupling cap will reduce your freq of oscillation which will be compensated by Cbank or vctl on VCO