It will be difficult to tune the VCO to an "exact" frequency, or to know the actual frequency, without using a PLL. Free running, the change in tuning sensitivy (MHz/V) across the band and manufacturing tolerances and temperature drift will cause trouble. But if your measurement system doesn't need to know the frequency value, and just tune to a resonance minimum/maximum and compare different resonators at that unknow frequency, then you might be OK without PLL.