Once again, when Jesus spoke these words, how many people at that time, prior to Him being glorified (John 7:38,39) had received the Holy Spirit and were baptized by one Spirit into one body? "the body of Christ?" (1 Corinthians 12:13). So IN ME is part of the metaphor of the vine, IN THE VINE not in the body of Christ under the New Covenant which was not yet fully established. It's interesting how you use a dispensational approach in your efforts to explain away the thief on the cross being saved through faith apart from water baptism, yet you ignore this critical truth above.
So in John 15, we see two kinds of connections with Christ as the vine (the merely cosmic which bears no fruit, (like Judas) and the vital which bears fruit - like the remaining 11 disciples). Without that vital union with Christ, there can be no life and no productivity. Those who profess to know Christ but whose relationship to Him is self-attached, He neither elected them, nor saved them, nor sustains them.
If Jesus had wanted to teach that there were true and false believers associated with Him, and if He wanted to use the analogy of a vine and branches, then the only way He could refer to people who do not have genuine life in themselves would be to speak of branches that bear no fruit (somewhat after the analogy of the seeds that fell on rocky ground and had "no root" in themselves" and produce no fruit Mark 4:17). Here in John 15 these branches that do not bear fruit, though they are in some way connected to Jesus and give an outward appearance of being connected to the vine, nonetheless give indication of their true state by the fact they bear no fruit. This is similarly indicated by the fact that the person "does not abide" in Christ (John 15:6) and is cast off as a branch and withers. If we try to press the analogy any further, by saying, for example, that all branches on a vine really are alive or they would not be there in the first place, then we are simply trying to press the imagery beyond what it is able to teach--and in that case there would be nothing in the analogy that could represent false believers in any case. The point of the imagery is simply that those who bear fruit thereby give evidence that they are abiding in Christ; those who do not, are not abiding in Him.