So what happens after 8 years? Do you need the money then? If so, I'd it needed because you plan on retiring around that time frame, or because you want to make a purchase with it, say a house?
More concisely phrased, what is your actual goal with this money (in how it's spent, not how it's invested)?
There are no differences in the rates at which Vanguard ETFs and Vanguard mutual funds are taxed.