Version of Go:
go version devel +b6ead9f Tue Jul 7 21:53:11 2015 +0000 linux/amd64

OS and architecture:
linux/amd64 (ubuntu 14.04)

What did I do:
Ran go get -t with vendoring turned on, expected it to only try to download test dependencies that didn't exist, like go get

What did I see instead:
It tried to download dependencies that already existed in the vendor/ folder

Reproducing:

Project hierarchy (located in $GOPATH/src/github.com/azylman/demo):

main.go
main_test.go
vendor/
  example.org/
    thing/
      thing.go

Files:

// main.go
package main

import (
    "fmt"

    "example.org/thing"
)

func main() {
    fmt.Println(thing.One, thing.Two)
}

// main_test.go
package main

import (
    "fmt"
    "testing"

    "example.org/thing"
)

func TestNothing(t *testing.T) {
    fmt.Println(thing.One)
}

// vendor/example.org/thing/thing.go
package thing

const (
    One = iota + 1
    Two
)

Output:

$ export GO15VENDOREXPERIMENT=1

$ go run main.go
1 2

$ go test
1
PASS
ok      github.com/azylman/demo 0.006s

$ go get

$ go get -t
package example.org/thing: unrecognized import path "example.org/thing"