It certainly could be an important consideration down teh road, but presently the OP has $18,500. That's $24/year or $2/mo. Maybe that extra cost is worth the simplicity to the OP, maybe it's not. Given how much Vanguard (and other disctoun brokerages) have changed in the last 10 years I wouldn't count on these expense ratios remaining static for another decade+.
OP listed contributing $18500, but made no mention of balance or employer match, so we don't know how much of an investment is being talked about. It may be different in this 401k plan but Admiral shares in my IRA account require $10,000 minimum, so investing 10% of a portfolio in VEMAX would require the portfolio value be at least $100,000.
If OP is starting from 0, contributing $18,500, and is subject to the $10,000 minimums, then I'd recommend starting with 100% VTSAX right now. To get desired international stock exposure, move to 70% VTSAX, 30% VTIAX upon passing $33,333 next year then consider the 70-20-10 mix once past $100,000 invested.