Very good questions here! I'm a big fan of Vanguard, Fidelity, and passive investing with index funds in general, so I might be able to help.
The short answer is that it all depends on the specific fund you are buying in to.
If you are buying Vanguard or Fidelity's proprietary low cost index mutual funds (the ones I like to invest in), they will not have front end loads or purchase fees when purchased through your Vanguard or Fidelity account. Therefore, you can purchase on a biweekly basis with no worries.
If, on the other hand, the funds you're purchasing have set fees per transaction, you might want to limit the amount of purchasing to a monthly basis or so.
You can see the fees for specific Vanguard mutual funds by clicking on any specific fund in the link below:
Vanguard Mutual Funds