VTI currently at 108.33 USD, meaning id have to pay 20% more to purchase with my CAD $
VUN is currently at 36.7 CAD, with its only holding being VTI. (only been around a year)
As forummm says, you have fallen into the trap of thinking that the share price of two securities can be compared to derive some meaningful conclusion. You cannot do that because the share price is not a meaningful quantity for comparison. What you want to look at is the "net asset value": the amount of assets that you are getting when you buy one share of the ETF.
At the time of writing,
VUN has a net asset value of 36.6846 CAD per share. Converted at the midmarket rate, one share of VUN buys you 29.43 USD of stocks. On the other hand,
VTI has a net asset value of 107.40 USD per share. That means that one share of VTI is (loosely speaking) equivalent to 3.65 shares of VUN.
At any given moment, the ratio of the prices may not exactly match the ratio of the net asset values, but that's because the market is not perfectly efficient (e.g. due to transaction costs). Assuming no transaction costs, it doesn't matter whether you buy a fixed dollar amount of VTI or VUN -- you are purchasing the same amount of assets. It's not the case that one costs 20% more than the other one.
If you can only buy an integer number of shares and you want to keep the largest ratio of your money possible in stocks rather than cash, then as Heckler says, you want to optimise for the smallest price per share, but don't confuse this as thinking that you are getting "more" assets per share -- you are not.
As for the alleged "garbage exchange rate", are you sure you know enough about forex to make such a judgment? You should probably think long and hard about whether you actually know (i) what the exchange rate "should" be, and (ii) why the exchange rate currently differs from your theoretical model. Only after you figure those two things out are you in a position to make such a comment and act based on it. That said, if after doing that analysis you conclude that CAD/USD is currently undervalued, the correct move is not to buy either VTI
or VUN (at least not without also buying some counterbalancing other securities), since both of those amount to going short on CAD/USD, because if CAD/USD appreciates, your US stocks will be worth less in CAD, which means the ticker price of VUN will go down, and although the ticker price of VTI won't change, you will get less CAD for your shares when you redeem them, so their value in CAD will also go down.