D-B diodes certainly can break down. But usually D-S
punchthrough comes first. And reliability will be
compromised well before either one, device drifts
(VT match, elevated "off" leakage, oxide wearout).
Presumably the VCO core is NMOS and a NMOS that
can barely take VDD-VS, may break down, enter
punchthrough or impact ionization at (2*VDD)-VS.
A LC VCO, by its L, can swing output above VDD.
Depending on the quality of your MOSFET model,
the D-B diode may or may not show its true operation.
Likewise D-S nonidealities. Modeling dudes can be lazy
and be whipped to turn out a normal-operation model
so quick that abnormal features get zero attention.
This is very normal organizational behavior, alas.
And it is why every circuit designer ought to try to
understand device design and device reliability.
Give a simple DC sweep simulation a try and see if it
looks realistic based on your understanding of device
parasitics and the circuit involving the true body
connection.