What would Nintendo have to give Vivendi to secure NX support, though? Based on Activision's support under Vivendi's ownership and Ubisoft's support under Guillemot's reign, it would be reasonable to suppose that it would be easier (i.e. cheaper) to guarantee support from a Guillemot-run Ubi than a Vivendi-run Ubi.
Again, though, there's a financial difference here between taking a shareholding in a company and paying (through advertising buy-ins or however it would be structured) for the support directly. For Vivendi to have any interest in a Nintendo offer, Nintendo would have to (directly or indirectly) spend at least enough to offset the cost of porting the games. This means a cost of perhaps tens of millions of Euros each and every year they want to retain that support, and that's actual expenditure, not an asset purchase.
Taking aside the figure of 1.3b for a moment, if Nintendo buys any number of shares in Ubisoft (or any other publicly traded company) that's not expenditure, it's an asset purchase, and the asset is worth the same amount at the time of purchase as you spent on it (by definition). Publicly traded shares also have an expected future value equal to their current value (plus government bond yields), which is to say that in the median case, owning a share of a company for a given amount of time is equivalent to holding the same amount of cash. The difference, obviously, between holding shares and cash is that shares are volatile, and that cash isn't. Holding shares increases your portfolio risk (both on the upside and downside), so the "cost" of exchanging cash for shares for a company like Nintendo is entirely down to their risk aversion.
So the question becomes, how risk-averse are Nintendo versus how strategically beneficial would a stake in Ubisoft be for them? They obviously aren't throwing money around like Tencent with acquisitions, but they were willing to drop $200 million last year on a stake in DeNA when they felt it strategically advantageous, so you'd have to say they'd consider shareholdings in the low hundreds of millions of dollars if necessary. If the actual cost of buying enough shares to gain a "kingmaker" position of being able to keep Vivendi out is anywhere near 1.3b (which was only a rough calculation based on current share price and zero other friendly investors) then you'd have to say it would be well beyond Nintendo's historical trend when it comes to risk-aversion. If the same thing could be achieved with a ~10% stake in the company, however, then it may be the kind of thing that Nintendo would consider.
As it is, if the Guillemots haven't been able to prevent Vivendi from taking over Gameloft, then I don't see them finding anyone willing to come to their rescue this time around.