Invest some cash into it. Start flipping RSGP. Buy RSGP from bulk sellers at a really low price, then just sell it for higher. GP trading is a quick way to get vouches.
An alternative would be to start some service threads, such as doing quests for people, leveling for them (botted or by hand, but you will most likely want to bot it), getting them minigame rewards, etc.
In addition to that, be a helpful/normal part of the Sythe community. Sure you can get vouches from doing things for people for cash/free, but people still aren't going to know you unless you post around the site once and awhile