now almost clear, thanks for clarification.
you can use rewriterule on virtual host(not .htaccess), and don't define any context. i.e., use virtual host level rewriterule to control all.
as before, define a web server external app:
the rewriterule in virtual host:
RewriteRule /vanguardia/(.*) http://test-proxy/$1 [P]
this should work as you expect.