I opened an IRA account with vanguard on 1/02/2016 and put the max contribution and put in 5,500. Since then the value has dropped to 5,000. I haven't done my 2015 taxes yet, can I add more to the fund? Is it correct to wait to file your taxes until the last do so you can maximize your amount of tax sheltered money?
No, you can't.
The limits are contribution limits, not limits on the value of the investment. If you contributed $5,500 for 2015, you are done contributing for that tax year.
You are free to contribute another $5,500 for 2016.
You can't directly contribute more. However, it seems that there is something you can do that can achieve a similar effect. You can withdraw your entire account (principal + earnings, though in your case that's less than the principal), and then contribute up to the $5500 contribution limit again. The end result is that you put in a net amount of $500, and the account ends up with $5500, which is what you want.
The first step is a return of contributions made for the contribution year before the tax filing deadline for that year. This kind of withdrawal is not subject to tax, and counts as if you never made the contribution at all. Since you are considered to have never made a contribution, you still have $5500 that you can contribute before you hit the limit.