Can I use "Signs of Evolution" to get a Vaporeon and a Glaceon from my deck?
No, you may not. Vaporeon & Glaceon are both Water types, and the three Pokemon retrieved by Signs of Evolution must all be different types.
Source: Plasma Freeze FAQ (2013-05-16), TPCi Rules Team (2013-05-16)

Relevant cards

Note: All applicable cards may not be listed below.
EX2_63
EX6_56
PL2_59