Do you mean a fare code? Or an exit row or extra leg room seat?
Wherever you are sat in economy and no matter the fare code or fare paid you still get the same level of service.
As gumshoe states if you want anything other than the cheapest non flexible ticket or the most expensive but flexible and upgradable ticket you need to call.
There used to be a search feature on the VS website where you could select a particular fare code.
Huzzah for International Jet-setting !