The code that you are using to get the transaction id looks correct to me. I can see that in the PP_ECO_VB.asp page near line 460 is where it identifies this variable:
elseif (testVal = "TRANSACTIONID") then
tValue = "PNREF"
So what you have should do the trick. If you can check the PP_ECO_VB.asp page in the eCart folder to see if you have this line of code in there. If it is not there then you may need to update your eCart. I created my example with the latest 4.5.1 release that is available on the site. Please check this and let us know what you find, if further debugging is needed I can open a ticket for you.