There is a lot of insight in @Ryan Rabideau post, almost all of which I agree with.
Valuation is tough on two fronts:
First up is actual rental rates. Just viewing the rates of competitors is akin to valuing property based on "For Sale" signs. As an owner/manager, I'm often providing concessions that impact value - preferred check in/out times, discounts, repeat-guest incentives, etc. These items are unknown to any competition shopping my rates page.
Second issue is the expense ratio of the VR. My home is a SF waterfront home and one of the more expensive rentals in my local market. I'm not selling days in my home, I'm selling a temporary luxury experience to my guests. With that comes a high expense ratio. My home better be in tip-top shape, hospital-clean, and exude an air of elegance. Additionally, my guests have paid a high cost - they rightfully expect premium service and occasionally demand some extra benefits, which comes in many flavors...but always exerts a time or cost on my bottom line. Simply put - my VR has a much higher expense ratio than a condo on the same block, and much higher than a bread & butter SFR.
Lastly - I see the greatest risk to our market is local policy. VR's are usually located in affluent areas, and many neighbors are power-players in the community. In my case, my greatest risk is the stroke of my HOA's pen, followed by local regulations, and then Acts of God. On a macro level, local acceptance of VR's certainly impacts value.