Your subject line suggests that you are using build tags. Please give us a complete example that we can use to recreate the problem. Thanks.

(Looks like issue #4906 but that was meant to be fixed.)

File A

package main

import "fmt"

func main() {
  fmt.Println("I do nothing")
}

File B with build tag

// +build mytag

package main

import "fmt"

func ButWhy() {
    fmt.Printf("but why?")
}

go vet output:

Expected output

$ go vet -x
/usr/local/Cellar/go/1.4.2/libexec/pkg/tool/darwin_amd64/vet a.go b.go

It's honoring the build tags, so it's not really a bug. But it should accept a -tags argument, like go build.
Will fix the subject of this issue to match.

-tags is one of the 'build flags' but we don't want all of them.

CL https://golang.org/cl/10692 mentions this issue.

CL https://golang.org/cl/10697 mentions this issue.

💃 Thank you @robpike!