A vouch doesn't require a trade behind it. Nor is it supposed to be a record of every trade someone has ever done. Rather, it is supposed to be an expression of trust. People hand them out like Cruel does candy on Halloween, in hopes that their vouch will earn one in return, but that isn't how it was meant to be. Since there is technically no rule against vouching over 'nothing' (staff and other trusted users often vouch based on how well they know a trader) it is hard to nitpick over what people can and cannot vouch for. GFX are admittedly an odd exception, but one that is deemed necessary - perhaps in part to preserve the quality of GFX to something more than a bunch of shops run by 5-minute vouch farmers.
Moreover, while a training vouch for 20-30 firemaking might be meaningless in a trade for a $500 account, the same vouch may be relevant to someone who is thinking about having the person train 50-55 fm. It is not up to us to decide what vouches you should trust and give weight to. That is up for each trader to decide for his or herself; that is why vouches are supposed to have links to the original post.