First, 20/40 corrected in each eye separately is good enough for a Third Class medical, so you're good on that for the time being.
Second, even if your right eye degrades to where it cannot be corrected to 20/40, but your left eye is 20/20 corrected and you are 20/20 with both eyes open, a Special Issuance based on a Special Medical Flight Test is not hard to achieve. Or, if you feel you cannot pass that 20/40 test with your right eye, you can just let your medical lapse and as long as you know you can fly an LSA safely, continue flying as a Sport Pilot.
So, if you're preparing for Private, and can pass a Private Pilot flight test, you really have nothing to worry about -- the SMFT for defective vision in one eye is pretty much just a portion of the PP practical test focusing on maneuvers relying on depth perception, and if your instructor thinks you can pass the practical test, the SMFT (if it is ever required) should be no problem at all. If you're preparing for Sport Pilot, and hold both a US DL and an FAA medical, stop worrying, and work on passing the practical test.