First off, vitamins add 10 EV's each to a specific stat, and cap off at 100 EV's per stat. So even if you reached the max limit of vitamins, Kingdra's stats are still not maxed out, so you will need to do EV training if you want to max any of its stats.
As for spread suggestions, it's a safe bet to just max out Special Attack and Speed, since Kingdra's only use is as a rain sweeper. There are more specific spreads you could use, but it's a hassle to get a Pokemon to a specific EV number in-game since it requires tracking it's EV's closer, and in the long run is pretty inconsequential for this mon.
http://bulbapedia.bulbagarden.net/wiki/Vitamin#Effect