There's quite a bit of a confusion here.
Everclear is a rectified spirit (pure grain alcohol)
'Rectified' refers to the method of distillation, not purity or source. 'Rectified spirit' is not equivalent to 'pure grain alcohol'. The latter can be the former, depending on the type of distillation. The former can be the latter if it's derived from grain (rather than from e.g. potatoes).
Vodka isn't anywhere near as distilled.
Russian state standard for vodka specifies use of rectified alcohol. This statement doesn't make much sense, as vodka is simply diluted ethanol. Before dilution it has been distilled just as much as any other rectified ethanol.
It's less pure, a lot less.
Alcohol used in high-grade vodkas typically undergoes extensive filtration beyond distillation. Industrial rectified alcohol will typically contain significantly more contaminants (e.g. fusel oils) unless it's UHP lab grade and is what's used in the crappy cheap vodkas.
Any time you create a liqueur you should use rectified spirits (Everclear/Crystal Clear/Golden Grain/etc) not vodka.
Depends on the liqueur. Some formulations call for grappa, some for brandy, and clearly will not tolerate a substitution with a rectified spirit.
