iRule- Need syntax help

The correct syntax is:

when HTTP_REQUEST {
    if { [HTTP::path] starts_with "/PasswordVault/auth/rsa" } { 
        HTTP::respond 301 Location "/"
    }

The missing part is the substitution operator (the square brackets) enclosing HTTP::path. Generally, when you want a command to retrieve a value that you will use, you enclose the command in the square brackets. Contrast, in your code, HTTP::path with HTTP::respond. With the former, you want the value (to use as the left operand of the starts_with operator). With the latter, you want the command to take an action, but do not need to capture the result (because there isn't one).