No, you have heard wrong.
There is a rare element, called decalithium but noone says it is only on Romulus.
It is just a rare, valuable element.
And one of the places they know to get it is the Kimben system which is where Nero and Spock go.
And as for the "moved to Vulcan" part, it's Vulcans that have developed the way to process it, produce Red Matter from it and use that Red Matter to create an artificial wormhole.
As for where the supernova is, i believe all we get is "Deep in Romulan territory" and that it can be seen from the roof of Spock's house on Romulus using some kind of telescope
For the record, Spock says 'isotope' not 'element' in the panel shown above. Made-up
elements (like
Stargate's Naquadah) are difficult to fit in with established science - there aren't any gaps in the periodic table, for one, and you have to have a reason why the element doesn't exist anywhere we can detect it, or been seen in particle accelerators. Isotopes we haven't discovered are more plausible. The name seems to imply this is an isotope of lithium, perhaps one with an atomic mass of ~10, given the prefix '
deca' (the usual weight of lithium is ~7 so a mass 10 isotope is on the plausible list).
Of course the problem here is the word in context is an obvious play on 'dilithium', which somewhat screws up the atomic mass theory