Abilities » Vessel of Life
When using Shedinja's "Vessel of Life" Ability is the underlying Nincada discarded, or does it get attached as part of the Pokemon Tool?
The Nincada is discarded; only the Shedinja card gets attached as a Pokemon Tool.
Source: TPCi Rules Team (2018-12-27)
Abilities » Vessel of Life
Can I use Shedinja's "Vessel of Life" Ability to attach it to a Pokemon that already has another Pokemon Tool card attached?
No, you cannot; the rulebook states "each Pokemon can have only 1 Pokemon Tool attached at any time". However, if there's an effect that overrides that rule, such as Genesect GX's "Double Drive" Ability, then you would be able to attach Shedinja to a Pokemon that already has another Pokemon Tool card attached.
Source: TPCi Rules Team (2018-12-27), TPCi Rules Team (2009-01-03)