That sucks!

My guess is there's some math you can do here, and the answer will depend on how much your money earns. If the expense ratio (say, 1%) is higher than $30 times the number of times you invest each year, minus the little fee Vanguard will take, Vanguard is still the better option.

Say you have $50000 invested earning 15% (ending the year at about $58k, not including any additions). 1% of that 15% is about $80. VTSAX has a .05% expense ratio, or about $4. You would only have to invest 3 times per year for Vanguard to be a bad choice. Of course, this all depends on the expense ratio of whatever fund you choose as well. The amount you invest is moot since that wouldn't change no matter where your money goes.

Say you have $200k invested earning the same 15%. You end the year up $32k. At 1%, that's $320, so you would have to invest 10x per year for Vanguard to be a bad choice.

It looks like if you don't have much to invest, and want to keep adding to it monthly go with a different fund where you pay no fee. If you have a lot to invest, Vanguard looks like the better option regardless of the $30 charge.

Since you're talking about adding $200-$300 each time, I'm guessing you don't have much to invest starting out. I think you should find the lowest fee fund with no $30 charge, and use this math to figure out when to switch to Vanguard!

Someone check my math here, but this seems right to me.