For an Integrated VCO, -40..+85 C working temperature needs a specialized process such as military grade or harsh industrial grade.
I don't think the standard processes ( TSMC, UMC etc.) are capable to work consistently between these temperature limits.Especially the process temperature characteristic is extremely important for such circuits as VCO.
In fact, if the VCO shows up consistent negative resistance under ALL temperature and process variations, it's necessary but not enough condition.Nobody guarantees that the VCO will absolutely work in ANY case.The process should be proven with thousands measurements otherwise 99 circuit works, one fails..