Requirement for “legal” is that the Mon is indistinguishable from one obtained via legitimate means.
In this case, as VC RSE isn’t released, and the mons you’re thinking of could not be caught nor received in VC FRLG, that means that there’s no legitimate way for you to get it.
Which in turn means there’s currently no legal copy you could make.
Something can be in the National Dex, but there being no legitimate way to obtain it; being in the National Dex isn’t a metric that “it must exist right now”.
Example 1: In Gen 3, RS launched first, prior to FRLG ColoXD release, thus one was stuck with only what was available in Hoenn Dex, with no hopes of getting Kanto starters.
Example 2: At the start of every Gen, Mythicals can’t be obtained despite being programmed into the National Dex, because the event isn’t released.
We’re at the start of the era of VC Gen 3. So not everything is obtainable at the moment. Hope that clears things up.
edit: also if you cannot wait, hack one in now and breed it (for mons that can breed). The one that was bred and hatched, while would still be illegal now, would likely be legal down the line.