This link will take you to a page which discusses some of the tricks that many parasites play on our immune system. It seems that VSG is not only used by worms but other invading organisms as well -- I was particularly interested to read that infection by Trypanosomes can cause a relapsing-remitting form of illness. At a guess I imagine the process goes something like this:
The body recognises an enemy epitope, (the pieces of protein which enable the body to recognise foreign tissue), and begins the process of building antibodies against it.
Once the numbers are sufficient, the invader presenting that epitope is attacked and destroyed.
However, by this time, using VSG some of the invading organisms have hidden themselves from the immune system by covering themselves with new glycoproteins. It takes some time to mount a defence against this "new" invader, during which both the antigen and antibody numbers rise, and the cycle begins again. Therefore there is a roller-coaster of fluctuating attack and defence, as numbers rise and fall, which results in the relapse and remission characteristic of some illnesses. That's my interpretation, anyway.
I was also interested to see that worms use the same technique not only to avoid an immune response, but to stop further infestation from competing worms... I'm not sure I understand how this works, but it would be a mechanism to avoid over- infestation.