in the restaurant when Isobel and Elena meet, Isobel is leaving and we see a boy passing behind Elena. Then Bonnie arrives and we see the same boy passing behind Elena again.
According to the vampire mythology, the longer one is a vampire the stronger they become. Isobel had only been a vampire for a few years. Neither Stefan nor especially Damon would have been remotely intimidated by Isobel or her minions. They had been vampires for around 145 years at this point and would have easily defeated all of them.