May I ask why you fear you will not be able to go back into a good law position if the worst happened? Just curious, not questioning. My sense is that you are a very accomplished individual and that a "failed" stint in VC would still be tremendously valuable experience.
I don't know anything about law practices though so would love to hear your thoughts.
Short answer is that the Bay Area corporate legal market is prone to over supply during tech downturns. I would be far more exposed to that as a job seeker particularly going back to a law firm as an M&A practitioner. This would be magnified by the fact that it will have been 4+ years since I would have been at a law firm and would need to build up my client base from scratch in a challenging market. Essentially I think the law firm re-entry route (for the type of practice I have historically had) would be closed.
I would have a better shot at an in house gig due to recency of experience, my network and the fact that inhouse lawyers don't need to generate business, but those jobs can be few and far between during downturns because companies fold, fewer new ones are scaling and requiring lawyers and existing positions tend to see less turnover due to market conditions.