Though Israel was the vine planted by God (Ps. 80:8), they had become rebellious and idolatrous; they were a counterfeit (Jer. 2:21).  When Jesus told His disciples He was the True Vine, He was emphatically stating to them that only identification with Him mattered.  Their nationality as Israelites was secondary to their relationship with Him.  We too, are to identify ourselves with Christ and abide with Him.  Everything else is to be secondary to our relationship with Him.