The attack outlined in the question does not work, at least as is. Still, SRP6a is very vulnerable to leak of server data, which is not part of the threat model that it addresses.
An attacker Mallory with knowledge of $v$ and $s$ (stolen from the server) can indeed masquerade as the server to the client and establish a shared key with the client. However, without the password, Mallory can not masquerade as the client to the server and get to the point where the server uses a private key. Further, if Mallory attempts to hijack a connection previously established with the server by the legitimate client, that will fail because that connection uses another private key than the one shared between client and Mallory.
However, with $v$ and $s$ stolen from the server, and pure SRP6a, an adversary can find a weak password by a dictionary attack (trying likely passwords sequentially starting from most likely), and then impersonate the legitimate client to the server. Technological progress ("Moore's law") tends to make SRP6a proportionally less secure from that standpoint. The only way around (beside not letting $v$ and $s$ leak, or hoping users will magically start using strong passwords) is that SRP6a is modified yet again by using a slow key derivation function for passwords (also known as password hash, password-based key derivation function) such as PBKDF2, Bcrypt, Scrypt, Argon2, Balloon instead of a fast hash.
Bottom line: when using SRP6a, if $v$ and $s$ being stolen from the server is considered possible, then the client software must implement a properly parametrized slow key derivation function for passwords; with the choice of that function, parametrization, and where the salt comes from not defined by any standard that I am aware of, including RFC5054 or ISO/IEC 11770-4:2006 (did not check the latest version). Correspondingly, practice is overwhelmingly to ignore SRP and send the password as keyed-in within an https tunnel (with the problem that a bogus certificate or a variation of the correct domain name that pass user