Live Houses (don't forget the earplugs):
Check out Turtle Island (raging tribal punk) here on April 7th: http://www.club-quattro.com/shibuya/schedule
Great venue - UK Subs and Forward play here on April 8th (FUCKING GO TO THIS!), and Framtid and Gauze play on the 21st http://www.antiknock.net/
Club Liner http://www.club-liner.net/
Make sure you print out a map to get to venues as the can be notoriously hard-to-find. When you're in the general vicinity, be on the lookout for punks on the piss outside from around 6pm. Drinking in public is permitted here.
Most of the great shops have closed and are run as distros out of people's apartments these days. A few still exist, however:
Disk Union stores in Shinjuku have tons of cheap punk/metal/hardcore vinyl and cds http://diskunion.net/st/shop/e_index.html
Killers is a newish shop I don't know much about, but could be interesing http://shimokita-killers.com/top.html
These shops always have tons of upcoming gig fliers as well.
Finding veggie food is tough in Japan, but in Tokyo there are tons of decent Indian restaurants. They do the best deals at lunchtimes where you could get a decent meal for around 1,000 yen. A lot of places in Tokyo have lunch buffets where you can eat as much as you can.
Kaitenzushi (revolving sushi) shops usually have quite a few vegetarian options and you can take whatever you want as it comes around. Go for kappa-maki (cucumber roll), shinko-maki (pickled daikon roll) and natto-maki (fermented soybeans roll).
Saizeria is a chain with cheap as Italian food. Many of these are open 24 hours and the have a few vegetarian options like pepperoncino pasta and a bunch of salads.
Freshness Burger are another chain with decent tofu burgers and mexican bean burgers for a reasonable price.
Pretty much all ramen sold in Japanese restaurants is made from pork soup.
I'm not a big fan of the Roppongi area myself (too many sketchy cunts), but it's alright for different kinds of food.
Tons of info here, but call in advance to make sure these places are still open before heading over: http://www.vegietokyo.com/info4vegie/restaurant/index.html
Check out the Yebisu beer museum, it's a fucking wicked place to get a midday beer: http://www.ratebeer.com/Place/empty/tokyo/yebisu-beer-museum/7615.htm
I'll post any other info as it comes to mind. Have fun!