As VAC is an automated system, user submitted bans are not accepted. As VAC uses a delayed ban system, the chances are that the cheater you encountered in-game has been flagged by VAC and will be banned in the near future. If you believe a player is cheating in a server the recommendation is that you contact the administration team for that specific server in order to report the possible cheater. This will allow the administration team for the server investigate the possible cheater in more depth and take whatever action they feel is appropriate.

You can also report a suspected cheater by following the information found in this FAQ