Don't look at the cost/share. You're not interested in HOW MANY shares you get but rather on the total DOLLAR VALUE of your investment.
$1,000 at $10/share of VTIVX = $1,000 investment.
$1,000 at $100/share of VFINX = $1,000 investment.
Nothing from this point on that would depend on the number of shares of each you own is going to affect your investment value.
Your returns (or losses) are based on percentage changes in the value per share. So if the VTIVX goes up 10%, your value is now $1,100 (less expenses). Same with VFINX, if it goes up 10% next year, your value is still $1,100 (less expenses). For all intents and purposes, the per-share values at the end of each day, quarter, year, etc. are assessed as a percentage gain or loss on the previous per-share value, not an actual dollar value gain/loss. Yes, the share prices change, but that's not what I look at because I just want to know what percentage did my total investment gain or lose that year.
Does that help clear it up a little?
I'm not sure if I can re-state a better way but I'm sure others can.