Based purely on numbers, VJ had a breakout season in 2008. In Zealots scoring he finished just outside the top 10, coming in at 13 overall at WR. He finished a few points ahead of guys like Welker, Bowe, Wayne to name a few, and no doubt was drafted much later than those three. I'll take that type production from my WR2 or WR3 any day of the week.
I didn't see all of the Chargers games, but the ones I did see, VJ was making plays. I saw him battle for some balls, and I also saw him make some great adjustments to make grabs. He had some pretty impressive stats on the season; 16 receptions of 20+ yards, 7 of 40+ yards, which was 2nd best in the NFL. 88.1% of his receptions went for 1st downs, which was tops in the NFL for a WR who caught more than 20 balls in 2008. He also averaged 18.6 YPC.
If VJ can keep his nose clean & stay healthy, I think he'll continue to improve as a big time NFL WR. With LT slowing down some, the Chargers may rely on the passing game even more in 2009, and that bodes well for VJ.

I watched as many Chargers games as I could last season but I am on the east coast so I missed several. My impression is that Jackson played very well... not sure if greyone just saw different games than I did or what.
IMO the biggest threat to Jackson duplicating his numbers is how many targets he will get. Last season:
The RBs got 138 targets... with both LT and Sproles back, I wouldn't expect that number to decline much, if at all.
Gates only had 92 targets... in the previous 4 seasons, his lowest total was 111, so I expect that number to go up.
Chambers had only 64 targets, a pretty low number for a starter. That could go up. Consider that he had at least 85 targets in 13 games with San Diego in 2007, despite arriving midseason. (I say at least because FBG doesn't list targets for week 20.)
Naanee and Davis combined for only 18 targets... will one or both of them have an increased role and thus more targets? I doubt it, but it's possible.
Floyd had only 37 targets. He played very well IMO, and made the most of limited opportunities, with 27/465/4 (17.2 ypc) on those limited targets.I think it is likely that Chambers, Floyd, Naanee, and Davis will combine for more than 119 targets this season.
So where does that leave Jackson's targets? Will Rivers attempt more passes this year? His 478 attempts last season was only 11th or 12th in the NFL last season, but that is deceiving since many QBs did not play all 16 games as Rivers did... as a team, San Diego was #25 in the NFL in passing attempts.