You mentioned having $1,500 for your IRA each year, plus you plan to make 401(k) contributions when you become eligible. In case you weren't aware, 401(k) contributions count toward the saver's credit too. Why not save at least $2,000 for this year to max out your saver's credit, then try to do the same next year? You might fall short of the $2,000 limit next year, but then again you might not! For all you know, you'll be able to earn a bit more or spend a bit less, and meet or exceed the $2,000 level.
As to your question about Vanguard's $3,000 minimum, you might be able to do one single transaction for both years as someone else mentioned. If not, just park your 2015 contribution in a target retirement fund (minimum $1,000) for a few days, then switch to your desired fund with your 2016 contribution.