He likely recommended Lady Vamp because she has relatively high base damage, as well as a high damage butt plug. I'm guessing he likes to drop Lady Vamp on turn one, and then Policewoman on turn 2 (so that she boosts Lady Vamps damage further with Silicone), in order to try to kill the opponent's first card right off the bat. If you can kill the first card in one shot, it can prevent the opponent from ever being able to build up a board.

But whether or not they're "the best" is arguable and situational. There are other blue cards with a similar skill set to Lady Vamp (such as the Proctologist, Hippy, Sommelier, and probably more), and also others with the Silicone skill like Policewoman (such as Sports Girl, Dog Lover, and probably more), that can also perform the same functions. And those cards might have different combos, which could make them better for different situations. And rather than dropping a Silicone card for turn 2, it can sometimes be better to build a combo instead. This is especially true if the opponent's first card has a high level whip skill. And I don't always like to combo the Lady Vamp, because she loses her Masochist skill, which prevents her from being able to grow. For comboing with something like the Thinker, I'd probably prefer to use the Proctologist instead of the Vamp Lady, because she retains Masochist, which can allow her to quickly grow to a point where she's guaranteed to be able to plow through whatever the opponent drops.

There are also different combat strategies all-together, in which some wouldn't really find Lady Vamp and Policewoman to be overly useful.

It's actually not fixed like that. It's just that the "Combo" tab is wrong. All the previews on that tab assume that both cards at at level 1. But when you actually use the combo in battle, it take the levels of both cards into account, and you will wind up with higher values than what is shown on the "Combo" tab.