A VDRL test can be used to detect new syphilis infections or follow the course of syphilis treatment, since the levels of anti-cardiolipin antibodies in the blood will change depending on how active a syphilis infection is in the body. Over the long term, latent syphilis infections can, however, lead to false negative tests. RPR is another non-specific test for syphilis that is used in similar ways, and both RPR and VDRL are known as nontreponemal tests.
There are also tests for syphilis that look specifically for antibodies to the organism that causes the disease - Treponema pallidum. These treponemal tests are somewhat less likely to cause a false positive diagnosis, but can remain positive even after an infection has been cured. They only determine that a person has been infected with syphilis at some point in the past, not whether or not they are currently infected.
It is important to know that VDRL is only designed to be used as a test for syphilis. If you are concerned you may have HIV or another STD, you will need to be tested for those diseases separately.
"Syphilis Testing Algorithms Using Treponemal Tests for Initial Screening --- Four Laboratories, New York City, 2005--2006" MMWR Weekly August 15, 2008 / 57(32):872-5