Yes, severely inbred plants (several generations of a plant breeding with itself) do get hybrid vigor, because ANY new genetic material results in a massive decrease in the expression recessive alleles.
It does not occur in animals because the level of recessive allele build up necessary to result in hybrid vigor results in animals simply not coming to term. To put it another way, an egg and sperm that inbred would just not result in a fetus. The only human populations that display anything close to hybrid vigor are Ashkenazi Jews and Pakistanis, both of which suffer from crippling genetic disorders due to generations of brother-sister incest for cultural and religious reasons. Every other group on the planet that has a population above 1,000 is in no danger of inbreeding (even Iceland is incredibly healthy, despite their Nordic induced autism about inbreeding).
In the case of Pakistanis and Ashkenazi Jews, hybrid vigor does not grant them super speed or stamina or something, it just means that their children are not cursed with crippling organ failure and neurological disorders.
