I wouldn't swap V-jax for Legend Larry Fitzgerald. I haven't heard of anyone saying Larry's that great of a card. And with V-Jax, you have an ultimate match-up against speedy CBs (the strength and slow speed to get behind the corner and catch the ball without dropping it). If you were going to replace Vincent Jackson with any other physical WR, then it would have to be MM Calvin Johnson, IMO.
Team MVP Delanie Walker is the best blocking TE in the game. TOTY Greg Olsen is a top 3 receiving TE in the game. I wouldn't necessarily sell Walker (as I'm not sure how many coins it would give you) because he could be used as a TE for his blocking ability and his receiving stats aren't terrible. But you certainly should take him out of the slot. I would put in a receiver like GC Patterson (+2 AWR, -1 STR) or GC Cecil Shorts III (+2 ACC, -1 STR). Either one of these receivers will significantly boost your team.
You should eventually upgrade PH Bell to Bo Jackson, as he is the best HB in the game. But for now you are fine, as Bell is quite suitable. PH Miller is great at FB and MM Vick is great at QB.
Your O-line obviously needs help, but that may take time depending on how many coins you have. MM Webster and Dierdorf are players you should target no matter what. They are the most affordable MM O-linemen. MM Larry Allen and Upshaw are too expensive right now, so I would hold off on upgrading at guard until you can afford either one of them. Since there is no new MM OT yet, PH Joe Thomas is the 2nd best OT in the game. No matter who you choose to buy first for OT, you should replace RTTP Team MVP Will Beatty first. RTTP Team MVP Anthony Davis is still a really good card and has 99 RBK. He should be the last OT you upgrade.
If you are less concerned about OVR, then MM Smith and Reggie White are not crucial. Instead, you should get Honors JJ Watt and place him in over Combine Clowney. You will want to replace Donald and Poe with MM Greene and PH McCoy. Until then, put Poe in the starting DT position over Donald.
Replace FO LaVonte David with PH Jamie Collins. He's the most affordable 99 LB. Personally, I like RTTP Levy more than I do FO David. I would leave Levy in the 3-4 LB position, but it doesn't matter which LB you choose to have there.
At MLB, you should play either PH DeMario Davis or Frozen Lambert. Lambert's stats may not be awe-inspiring, but his 100 PLR is the best of any defensive player in the game. PH Davis is like MM Pat Willis-lite. Willis is still too expensive, so one of the other 99 OVR MLBs will have to do.
IMO, your defensive backfield is solid. I would save up your coins to purchase more top tier defensive backs in the future. Your targets will be MM Deion Sanders and Legend NTL. You'll eventually want to upgrade to Harrison Smith at safety, but I've found that a boosted RTTP ET3 can be just as effective (he's got so many picks in H2H). From there, you would determine whether you want ET3 or Patrick Peterson in your dime (once you have all the other great DBs). But for now, leave your offensive and defensive backfield are the strongest parts of your team.
This is my advice for now. A lot of this depends on how many coins you have. PM me with any more questions on Madden Mobile, or if you need coin making tips.