Yay, i hope you will enjoy it :)
If you have a car you have to check out Schillinger - http://www.schillinger.co/
from vienna it takes about 45 minutes to get there by car but is totally worth it. It is the best. If you go there you should make reservations.
Schillinger opened a fast food restaurant in vienna - Swing Kitchen, which is also very nice: http://swingkitchen.com/
i can also recommend st. josef naturkost for lunch (vegatarian) but you should know that it is very casual and more like a canteen than a fancy restaurant: http://www.yelp.at/biz/naturkost-st-josef-wien-3
dr. falafel (naschmarkt) is great for a snack or for lunch but it might be cold in october - there is no indoor seating, as it is just a booth...
Maschu has also very nice falafel: http://www.maschu-maschu.at/
- the one in 1030 is the nicest: http://www.maschu-maschu.at/maschu-1030/
all-vegan sushi bar: http://www.pirata-sushi.com/
also quite casual.
Wrenkh (have never been there but i plan on going soon, I walk past this place almost everyday and it seems really nice): http://www.wrenkh-wien.at/
yamm (vegetarian) has nice vegan cakes, so i would maybe go there for coffee and cake. the food is mostly vegetarian, the vegan dishes are (at least in my opinion) not very special most of the time: http://www.yamm.at/
If you are into raw foods: http://www.dancingshiva.at/
i have never been there, but friends are raving.
There is an all-vegan bakery which i have not been to but should be mentioned: https://www.facebook.com/nomnomveganbakery
There are two vegan supermarkets: veganz (https://veganz.de/de/maerkte/wien/
) and maran vegan (http://www.maranvegan.at/
if you like it sweet you should definitely buy something from bernd's welt (http://berndswelt.at/
) e.g. at veganz or at a spar market. some spar markets have berndswelt's products.
Radlager is one of my favourite cafes (and also sells bikes): http://www.radlager.at/
blueorange is another cafe (http://www.blueorange.co.at/
) it has some very nice vegan snack-/lunch- or breakfast-options and most of the time also vegan cake.
if you are into asian fake meat dishes: vegetasia http://www.vegetasia.at/
(Medaillions à la Vegetasia are really nice)
Not a must-go-to but can come handy if you are in need for coffe with soymilk: Ströck is a bakery chain, they have some vegan options and soy milk for coffee - http://www.stroeck.at/
Naschmarkt is nice for a stroll (better not on a saturday though, as it is really crowded).
Schönbrunn is also a "must-stroll".
What are your personal interests/needs etc.? Are you into art? Do you speak/understand german? In which district will you be staying?