Viglink does not strip existing affiliate codes.
There are a couple of companies in this space, viglink, skimlinks, etc. They all operate on the basis that they sign up to every affiliate scheme and they become the single publisher with which the merchants have a relationship (or more likely, the merchants use a marketplace that puts all merchants together so that they can be found).
For any affiliate program that viglink/skimlinks is signed up to, should someone click through from your site a 45 to 60 day cookie is usually set by the affiliate provider to show who the last publisher is that made the referral. Should a subsequent purchase be made viglink/skimlinks is credited with the sale and receives the 3.5% > 15% affiliate payment.
Catches (there are some)...
1) The cookie. Only the last web site to set a cookie gets the affiliate reward. In this case it means other sites actually providing value and setting affiliate click-thrus are going to get their cookie hijacked by posterous using viglink. So this hurts niche sites that really rely on the affiliate schemes.
2) The fee. Viglink and SkimLinks both deduct a 25% fee for this service. That's pretty damn high. They are VERY profitable.
The benefits...
1) As a publisher you get 75% of the money that is currently left on the table.
2) You didn't have to sign up to every damn affiliate scheme... actually you'd be declined anyway as whilst a merchant will deal with the big boys they tend to only like smaller guys who stay in their knowledge domain (if you don't run an insurance content site, you don't get the insurance affiliate scheme).
Basically viglink and skimlinks are simplifying the whole affiliate thing by doing for the publishers exactly what the merchants already had in places like Commission Junction... which is to provide a one-stop-shop.
So... merchants get found via CJ, AffiliateWindow, etc... and publishers get signed up to all merchants via Viglink, SkimLinks, etc.
What has ultimately happened is that now there are 2 intermediaries between the publisher and the merchant, both of whom take a slice.
I don't blame posterous for doing this, it is just money left on the table. But the cookie hijack will hurt smaller sites for whom the affiliate scheme revenue is a bloodline.
All they need do is to act like another viglink or skimlinks and to record the clicks made per publisher using their platform. Then offer the publisher 50% of the revenue.
This is then an incentive to use Posterous beyond all of the ease of use. Anything you posted that included a potentially monetisable link would earn you revenue.
Now, this isn't as lucrative as just doing it yourself. But to all of the creative people making cool content who are lost when it comes to affiliate schemes this would be a killer feature.
Spammers aren't entirely stupid, they'd go register direct and do what they usually do (set up little spam bait sites) as that would be way more profitable.
This would help small publishers more than it would assist spammers.
I only recently found out about these services, and my first thought was "cool!" But now, it occurs to me that they're built on very shakey ground. They only work if they're not popular. If they start becoming widespread, then the affiliate programs will have to lower (perhaps drastically) the commissions offered to compensate for the fact that suddenly every link is an affiliate link.