possible solutions:
if you have a lot of proxies and displacement, try to
increase your dinamic memmory limit on vray system panel...(400 mb default), put 1200 or something like this...
if you have some proxies, try to put the original meshes, will take more ram, but the calcualtions will be faster...use the proxies only to save ram..
take in account that not allways is possible to use all the ram, sometimes the rendering times are depending on other calculations, like subdivisions, light quaility, noise etc.....more ram memmory not allways made this calcualtions faster,..
wich processor you have?, how is the scene?, how much lights...
if you put the vray settings i can help you, but in this way looks like a lottery..