I agree you may run the risk of worrying about money too much. My thoughts are to certainly shorten the trip - even by just a couple of weeks - and def drop Tokyo. I am a backpacker myself, meaning I gravitate towards the cheaper end of most things most of the time when travelling, and I spent over £250 in just 2 days in Tokyo in 2007. I stayed in a capsule hotel in Shinjuku that was about £35 a night, travelled around by train and taxi, ate quite modestly etc and my money just seemed to disappear.
On the same trip I spent just over 2 months in India and about £1300 including 3 domestic flights, and a night or two in mid-range hotels not cheap guesthouses. I could have done it for a fair bit less.
However, I personally don't like going away with the bare bones daily budget and no room for manouevre because I like a drink from time to time, don't mind a days mountain biking here, a 3 day trek there, a scenic flight over mount Everest...etc etc. Simply eeking out an existence in various parts of Asia is only an exercise in money management and not exactly a case of freeing oneself from the shackles of daily life.
If you just went to India there is a heck of a lot to see, and it is cheap (£15-20 a day p.p. would do it).
I'm an old cynic, and fairly well-travelled in Asia, but I would drop Malaysia right off the bat. I spent 3 weeks there in 2005 and it is OK but didn't do much for me after Indo-China. 6 days in Vietnam means barely scratching the surface. Hong Kong is cool but a crappy room in Chungking Mansion is over a tenner at least - I would budget at least £30 a day for HK.
How are you getitng from each country to the next - are flights already paid for, leaving you/your girlfriend with £700?
How about this? Save up a touch more and then....
India for 6 weeks and then Nepal for a couple of weeks? I'd say £1000 each excluding flights would nail that. Cross from Varanasi up and in to Nepal by land and back the smae way - rtn flights into india. job done.