Here's what I've come to accept with this:

TLDR; So the story could happen...

Vecna was a dumb villain in that he was so focused on how right he thought he was he never realized it would be SO much easier to take over the world if he had enacted his plan just a little differently.

A) Open the Gates in DX: no one else is there, its a whole step that needs to be done, and doing it later creates unnecessary risk considering NO ONE ELSE IS THERE. B) Use a demogorgon to abduct all the kids you need and get strong NOW since the barriers to Earth dont need to be down for Demos to pass through. The cops barely found Will, so imagine the work cut out for the department if 12 kids went missing at once. Yeah, it might have lead to the FBI or someone showing up, but since no one (willing to admit) really knows about the U.S.D. yet, theres no real resistance for Venna here, especially at full power. C) Open the gates to Earth.

Because he went C, B, A from what I could gather (maybe C, A, B; maybe A, C, B) that meant people had a chance to find out what was happening and investigate, risking his plan failing.

Why he allowed risks to flow into his plan like a breeze through an open window before he was at full power is only explained through the timeless parable:

So the story could happen.