A godep replacement, in 35 lines

po.go

package main

import (
	"flag"
	"log"
	"os"
	"os/exec"
	"path/filepath"
	"strings"
)

func main() {
	if _, err := os.Stat("_vendor"); os.IsNotExist(err) {
		if err = os.Mkdir("_vendor", 0755); err != nil {
			log.Fatal(err)
		}
	}
	flag.Parse()
	args := flag.Args()
	cmd := exec.Command(args[0], args[1:]...)
	cmd.Stdout = os.Stdout
	cmd.Stderr = os.Stderr
	cmd.Env = os.Environ()
	for i, path := range cmd.Env {
		if strings.HasPrefix(path, "GOPATH=") {
			abs, err := filepath.Abs("_vendor")
			if err != nil {
				log.Fatal(err)
			}
			cmd.Env[i] = "GOPATH=" + abs
			break
		}
	}
	cmd.Run()
}

You probably want _vendor instead of vendor: Go auto-ignores directories with a leading underscore.