Are vanity addresses as secure as an average randomly generated address?
In theory they are because private keys are generated at random until one is found who's address fits the criteria. For example, for the address I posted above, vanitygen generated about 50 million private keys before finding one whose address started with "1Frac5" (it got lucky and found one quickly).
However, the security of the address depends on vanitygen's ability to generate private keys that are as random as keys generated by other applications.