The first two questions have already been answered so I'll try to address the VI(U) & VTI vs. VXC issue.
First off is that the fees for VXC are higher than the fees for VXC and VTI and it's Canadian listed version VUN. So you could save between 5 and 10 basis points right there.
Next, international markets do not all behave equally and they certainly do not move in lockstep with US markets. VXC is a cap weighted index so when US stocks are valued much higher than other international stocks (like right now for example), VXC will actually hold more of its value in US stocks. This is counteractive to the point of maintaining a balanced portfolio where on an annual basis you sell high and buy low to re-balance your portfolio. At some point the extra fees and trading costs are no longer worth it, but from a pure performance perspective it makes sense to break up your portfolio into multiple categories to achieve a greater benefit from re-balancing. The attached image can put it into a visual perspective a bit better, but if you look at returns for each category, more often than not an asset class that performs very well one year performs not so well the next.
So rather than VAB, VCN, VXC a better option could be:
VAB: 20%
VCN: 20%
VUN: 20%
VE: 20%
VA: 20%
I would only say it is worthwhile doing this for larger portfolios. If you're portfolio is worth less than $100k, it is simply not worth it to re-balance annually with multiple trades worth a few thousand each.