A country with Internet censorship technology in place (like Iran) is easily capable of blocking VPN Internet addresses that it knows about, as well as prohibited website Internet addresses. Since it is relatively easy to identify VPN Internet access addresses simply by installing or signing up for a VPN service and then monitoring the packets being sent to the VPN, blocking of VPNs in tight-censorship nations should not be surprising. There is no Internet solution to the problem, apart from a VPN supplier willing to intelligently hop a user's connection frequently among a wide range of candidate addresses, and constantly change that array of possible addresses over time. Such a scheme is complex and costly, and in the end, not totally free from blocking.
As @leocg noted, there's really nothing a browser maker or a targeted website can do.