Haven't had to worry about this much with tax my tax advantaged accounts. But I have started up a mutual fund with Vanguard, and I was curious if there are any tax related concerns with investing a set amount every couple weeks, vs one lump sum payment a few times a year.
If you're just purchasing shares in the fund, the interval at which you purchase them shouldn't matter directly for tax purposes. Since you're not selling any shares in the fund, and I'm assuming you're reinvesting your dividends, this position shouldn't generate any taxable income.
However, when you do choose to sell, you'll likely have a different cost basis than if you invested the money all at once. Investing at periodic intervals is known as dollar cost averaging (DCA), and the usual assumption is that it lowers both risk and return because you accumulate your position over time and at different prices.
DCA also affects which parts of your investment qualify as long-term or short-term capital gains; if you opened your entire position at once, all of your shares would have the same term status, but if you accumulated your position over time and decide to close all or part of your position within a year of any purchase, some portions of your gains may not qualify as long-term capital gains. Sales like this follow a first-in-first-out-procedure, so the shares you bought first are sold first.
This question on DCA has some good information that might help you too. Note that Vanguard published a study that recommends investing the entire lump sum at once if you have the entire amount available. If you're simply investing part of each paycheck, Vanguard recommends DCA as a "prudent way to invest."
Mutual funds do have different tax implications than ETF's because of the way shares are redeemed
Although this isn't a tax issue, make sure you're aware of any commissions or annual fees Vanguard will charge you if you choose to trade non-Vanguard funds or other securities. These vary depending on the type of account you, so look at their information to see what applies to you.
There are different types of cost basis methods: average cost, FIFO (First in, First out), and specific identification.
With specific identification at the time of selling you specify shares from which purchase you are selling. This is the most complicated but can lead to the lowest tax bills.
If you invest every couple of weeks, you will have more transactions to keep track of and more complications if you decide to use the specific identification.