I highly doubt Russia was exporting M1870/87 Vetterli-Vitali rifles to the U.S. at ANY point in time. It is well documented that Russia sent many rifles, and the types of rifles were well recorded too. You should look into the book Arms for Spain by Gerald Howsen. Another source of information would come from vintage firearms magazines from the 1950s and 1960s. Here's an example from a 1956 issue of Guns Magazine:
As you can see, the Vetterli-Vitali is being sold along with the M1874 Gras. The Gras is often found marked "Made in France" and with Russian markings. I have one such example. These rifles are also attributed to the Spanish Civil War, and since they commonly have Russian markings, it would be logical that they weren't exported from France, but instead from Spain. So in my opinion, it would be highly unlikely that two types of rifles, originating in two different countries, both being sent to Russia in WWI, both being marked in the same manner upon export to the U.S., and both having similar Russian markings, would have been exported from different countries (i.e. the Gras being exported from Spain, and the Vetterli-Vitali from Russia). Here's an example of a Russian marked M1874 Gras:
There ARE Vetterli-Vitalis that don't have Russian markings but are still marked "Made in Italy." I have an example of one of these rifles as well. As well as I can tell, I'm assuming these rifles were given directly to Spain by Italy, or were possibly used by Russian forces but never marked.
Another route of investigation you may follow would be about a man named Sam Cummings, and the company of Interarms which originally found and imported these rifles to the U.S. One of his greatest caches of firearms was in Spain, and it would seem the Russian marked Gras and Vetterli-Vitali rifles came from this lot.
Hope this helps.
EDIT: Just wanted to point out that in the ad above, there are also M91/30 Mosin Nagants being sold. Notice the reference to them being confiscated from "Communist revolutionaries" and that the sale of these rifles in no way aids "any iron-curtain country." These vague references seem to be pointing to these rifles coming from Spain. It's possible these rifles MIGHT have come from Finnish sources, but when coupled with two other types of rifle commonly attributed to Spanish Civil War use (the Russian marked 1874 Gras and the M1870/87 Vetterli Vitali), all the evidence seems to point that these 3 rifle types in this ad, at this time period, are from the Spain.