Not much known about Thomas in the earlier issue of the W.I.T.C.H comic except that he left the family and divorced by his wife, Susan Vandom. She revealed to Will that they left Fadden Hills because of him.
When W.I.T.C.H. attempt to enter Kandrakar, Will had a dream where her mom broke up with Dean Collins and her father returned to the family in Issue 009: The Four Dragons. He made his first appearance in Issue 024: Trust Me where he was reunited with his family. He initially showed friendliness and begged forgiveness for leaving the family, he also spent time with his daughter and reminisced about the past before Thomas left the family.
However, in Issue 026: Blackmail, he revealed his true colours as he hired a private detective, Harvey Slimverick to spy on his wife. The detective managed to take incriminating photos with Susan and Dean which allowed him to start a custody case over Will, which was presided over by Judge Theresa Cook. He blackmailed his wife to give him money to pay off his debt or she will lose the custody case with the detective's pictures hampering Susan chances of winning the custody case. The confrontation was unwillingly witnessed by Will who transported to her room.
Kadma revealed to Will and Taranee that Thomas committed bad deeds to obtain more money. Despite having a normal life, he wanted more and make risky investments which costs him a lot of money, essentially making him a gambler. Susan was forced to pay off his debt and Thomas still wanted more money by commit deeds like selling Susan's parents house for example.
Although the chances of Thomas winning the custody case is higher, he withdrew the case as Kadma offered him more money than he needed in promise to leave the Vandom family alone. He kept his promise and never seen again in the rest of the comic series.
In the TV show, he is called Tony Vandom, is divorced from Susan Vandom, and is engaged to Serena. He appears in Season Two of the TV series. Tony is rather wealthy owns a large apartment and has accesses to the building's swimming pool as well as a convertible. It is unknown what caused Tony and Susan to divorce in the animated series though judging by their fighting it is implied it came from Thomas's less responsible side. However, he is shown to have a far better relationship with his daughter Will than in the comics and actually cares for her. In N is for Narcissist, he offered to let Will and her friends skip washing cars for a ride in his convertible. Much like Luba in the Television Series, Tony is a much nicer person he even has a good opinion of Dean Collins.