Vanessa is 30 years old officially.
When Vanessa was being created, SNK wanted to drastically increase the average age of all their fighters from previous games. Prior, they had introduced extremely young characters, such as Bao, Li Xiangfei, Chris, etc. They wanted to put in older characters and thus, characters such as Lin and Seth arrived along with Vanessa's debut. When they decided to introduce a new female fighter, they wanted to create one with a more adult and mature appeal, unlike the more "girly girl" females the series had before-hand. (Such as Yuri Sakazaki, Athena Asamiya, Mai Shiranui, etc.) To enhance Vanessa's 'adult appeal', she was also designed as a mother - she has a 4 year old daughter - and as a wife, though her husband was killed in battle when his mercenary organization went up against the NESTS organization. She was given a business suit-look to complete the style - though she wears her vest unbuttoned to enhance her sex appeal.
Vanessa has appeared playable in the following:
KOF '00, KOF '01, KOF '02, KOF XI, KOF Neowave, KOF '02UM
She did, however, appear prior to KOF '00 - in the Dreamcast port of KOF '99 as an unplayable striker. She has not appeared in the Maximum Impact series. (A feasable reason would be that her racy outfit would not go down well when rendered in 3d)
She has also appeared in:
SvC Card Fighters Clash 2
Svc Card Fighters Clash DS
Crimson Viper, from the Street Fighter series (introduced in SFIV in 2008), shares alot of similarities with Vanessa. Both are single mothers with young children - Vanessa has a 4 year old (un-named)daughter, Viper has a daughter named Lauren (though her age is unknown, it is likely she is around 4 as well, as Viper is close in age to Vanessa). Both also wear somewhat similar business suits (though Viper wears a business jacket as well), and both have red hair. They are also both single - Vanessa's husband died in the line of duty, while the whereabout's of Viper's husband is unknown (a common theory being that Charlie Nash, who was killed by M. Bison, was her husband).
She is the first playable female fighter in the KOF series that is in her thirties. (As of official age)