While I agree with most of what you have said, saving for and investing iteratively in the funds means that for the 100 weeks while you are executing your plan, you are way out of balance and thus exposed to more risk than you may desire.
You should think about finding a balanced (or target retirement) fund with your desired allocation. (Target Retirement funds, don’t have to be held in a retirement account.)
Another option would be to use a service like sharebuilder, zecco, or other similar ones to build a balanced portfolio from day-one using ETFs. The fees are low (similar to vanguard) and you could buy little bits of each ETF every month to keep your portfolio balanced.
