Prometric and Vue are two different companies offering the facility of testing. You can take tests at Prometric or Vue. What IBM means with this answer is that if there is no center for Prometric in your city but a center from Vue, you can exchange your voucher, which is originally meant for Prometric, for Vue.
However, in your case, if your IBM certification center is not listed in www.2test.com website, i would seriously suggest to check if this center you have is an authentic and authorised one! Maybe thats why he said this voucher was not valid at his center!