Orchid Focus Bloom (there is no such things as a bloom booster - read this:
Plant Boosters) is a liquid that is 1.9% N, 2.5% P, and 2.6% K, so you are mostly purchasing water.
For 125 ppm N - probably way too low for vandas - you'd need over 6 ml of the stuff per liter. And at £10 per liter, that's double what I charge for a liquid fertilizer that is more than double the concentration - i.e., 4 x the cost per use.
The key to getting vandas to bloom is plenty of light and plenty of food, and you can use almost anything (short of super high-nitrogen formulas).