Roselia gives 2 Special Attack EVs. (Near the bottom of the page, just after the "Pre-Evolution Moves" list, if you're curious.)
Using the
4th Gen IV / Stat calculator for assistance, I can tell you the following:
-Your Palkia has a Special Attack IV of 10.
-You gave it 10 vitamins to raise its special attack ; only with an IV of 10 and 100 Special Attack EVs would a level 47 Palkia have a Special Attack of 150 before vitamins but a Special Attack of 162 afterwards.
-When it leveled up, it had at least 244 EVs in Special Attack. 252 is the max. (Well, actually it's 255, but the three points after 252 don't increase the stat.) Otherwise, its Special Attack would have been below 183.
-When it leveled up, it has less than 252 EVs in Special Attack. Otherwise, its Special Attack would have been 184, as high as you'll be able to get it.)
If you've downed *any* Roselia since then (assuming you're still wearing the Power Lens) then the Special Attack EV should be maxed. Palkia's Special Attack should be the following stats at nearby levels:
Level 49: 187
Level 50: 191
Level 51: 195 (If you have this value, you have successfully maxed out the Special Attack.)
Palkia's level 100 Special Attack will be 378.
Edit: This is all assuming I've understood the order of events you've given.