I'm not worried about the vachetta getting wet, only the opening! I know it closes under your arm, but water will still get in, unlike with a zipper.
I have a noe and I have no issues when its raining, and it just ties cloesed.
In fact I had a piano bag that i use to carry a lot, never zipped it up. I was in the pouring rain walking around at an antique show, nothing got wet The bag did and dried nicely .. No worries get the BV.
I think because the BV goes in on the sides and tucks nicely under your arm you'll be just fine.. I think you'll get more use out of it then the speedy.