VCP is a Certification requiring a proctored exam. Designed for a technical person to validate thier knowledge to prove they know the tech and how it works at the nuts and bolts level.
VSTP is an Accreditation requiring about 7 hours of your time to go through the online courses from the Partner Unversity and complete about 7 short quizes with no limits on being able to refer back to the material or redo the test. It provides some VMware Partner and licensing information along with and high level overview of the technology. The idea is to skill up the pre-sales staff of an organisation so they are better able to talk turkey with customers before bringing in the VCP (or beyond) to deliver on the sale. If the course material is to be believed someone with a VSTP is meant to be a vastly better sales person raking in 4 times revenue in sales....
From the perspective of the VSTP being of value to someone pursing a VCP certification (speaking from a vSphere 4 perspective at least) its handy as free course for high level revision of the very broad topics and a closer look at the licensing from the point of view of what different editions provide but its effectiveness is limited. Unless you have the time to do this as well as more technical study you'd probably get more out of spending that same time playing in a lab.
Then again if you have to get a VSTP and/or a VSP (fully sales focused acceditation) to meet VMware Partner requirements then doing the VSTP and VSP first might cover some material you missed and also give you more depth from a licensing perspective before you go for the VCP exam. Having all 3 "qualifications" MAY make you more attractive to a VMware Partner that is short of meeting the requirements expecially if part of the role is sales based.
