Now, the Macho Brace ONLY reduces the speed while in battle (like if you have been attacked by a Scary Face or something prior the fight), but still collects the EV for speed, thus, NOT modifiing the final value of it. This means that if your poke has 120 Speed points and it´s wearing the macho, his speed drops to 60, BUT, let´s assume that he defeats -something that gives 3 Effort Points for speed-, he´ll still receive the 6 points for Speed, so, if he grews up ,let´s assume that his speed climbs to 162 thx to the 6 EV previously received, but if he still haves the macho, the "in battle" speed will be of 81, going up to 162 after you remove the macho.

In other words, as far as i know, Macho Brace it´s not perjudicial at all...

Now, about "the it won´t have any effect with vitamins", that is because vitamins in RBYGSC boosted the stat experience of the Pokey, instead, now each vitamin boosts the Effort Points/Value of the matching stat by 10 points. This mean that if you want an EV of 88 in attack, you´ll have to give your poke 8 Protein and then fight 8 pokes (max) for the remaining points.
BUT, you cannot give a poke more than 10~11 (this means between 100 and 110 EV in that stat) vitamins per stat, neither being allowed to give him more than 51 Vitamins (or the top of 510 EVs).

Hope that helped...and that someone understanded it O.o