Vanguard Lazy 3 question

Hey guys, first time poster.

I just opened a Roth IRA with Vanguard and I am trying to allocate my portfolio. I want to do something simple to start - 50% Vanguard Total Stock, 25 % Vanguard International Stock, & 25% Vanguard total Bond.

My question is - is there a way to set up my first purchase with percentages like these? Or do I have to manually allocate each purchase for the first time? I'm seeing that you can set up automatic investment, with a percentage option to each of the three I've outlined - but the auto investment can only be set up once I've purchased an ETF, Mutual, etc.

Any help would be appreciated - I hope this makes sense.

LJ