Yes, both of these points are correct. If you are connecting a reference clock to the Vivaldi Clock, it has to be more accurate than the Vivaldi Clock in the long term for it to have any positive impact. So, the accuracy needs to be higher than +/- 0.1 parts per million over time.
Jitter (and subsequently phase noise) on a reference clock going into the Vivaldi Clock is largely irrelevant as in essence the Vivaldi Clock doesn’t respond to fast changes on the reference input - it has a slow acting PLL (also known as a narrow bandwidth) so any high frequency changes, i.e. jitter, are filtered out. Within limits of course - if the jitter is extreme enough it may cause the clock to fail to lock to the incoming signal, although one would like to think this would be rare on an external master clock!
It is also worth noting that the kind of clocks likely to produce this level of accuracy are likely to be atomic clocks, and these are usually a bad idea to connect directly to audio equipment such as a DAC as despite being incredibly accurate over time, in the short term there can be a fair bit of variation as the system tunes around the target frequency - jitter. Not great in terms of the DAC, but when fed to another master clock with a slow acting PLL the high long-term accuracy reference can have a positive effect.