The US nonresident alien tax (codified at 26 USC § 871, and subject to any modifications contained within tax conventions (treaties)) applies to most income received by a nonresident alien of the US from sources within the US.
VUN is a resident of Canada, and thus it is a nonresident alien of the US. Thus, when VUN receives income from US sources (such as dividends from the US stocks held by it), VUN has to pay the nonresident alien tax. VUN then distributes the net income to its shareholders.
Since the distribution from VUN is not income from a source within the US, the US nonresident alien tax does not apply to that distribution, regardless of what account it is held in -- even in a taxable account, that distribution is not subject to US nonresident alien tax. However, since the IRS took 15% of the dividend payments received by VUN, the distribution you are receiving is still only 85% of the gross dividend, regardless of whether VUN is held inside an RRSP.