VyOS provides policies commands exclusively for BGP traffic filtering and manipulation: as-path-list is one of them.
set policy as-path-list <text>
Create as-path-policy identified by name <text>.
set policy as-path-list <text> description <text>
Set description for as-path-list policy.
set policy as-path-list <text> rule <1-65535> action <permit|deny>
Set action to take on entries matching this rule.
set policy as-path-list <text> rule <1-65535> description <text>
Set description for rule.
set policy as-path-list <text> rule <1-65535> regex <text>
Regular expression to match against an AS path. For example "64501 64502".