That is a common misperception with new trainers. What you fail to realize is that while you can use vitamins to increase your pre-evolutions to make their stats higher, you can also do the same thing with fully evolved Pokes to widen the gap once again. I am not understanding how this thread keeps living. Let me throw out a comparison to you, using a Lv100 Treecko and a Lv100 Sceptile. Let's even cheat and give both Pokes 31 IVs in every stat. Both Pokes also have had no EV training and have neutral Natures. First, here are the stats for Treecko:
Lv100 Treecko
HP: 221
Atk: 126
Def: 106
SAtk: 166
SDef: 146
Spd: 176
Now, let's compare them to the stats of a Sceptile:
Lv100 Sceptile
HP: 281
Atk: 206
Def: 166
SAtk: 246
SDef: 206
Spd: 276
No comparison, whatsoever, Now, let's pump our Treecko full of steroids......errrrr I mean "vitamins". Remember that you can only feed vitamins to a Poke until that stat has 100 EV points and that it takes 4 EV points to increase a stat by 1 point. That said, keep in mind that a Poke can only have a total of 510 EV points added to their stats. That means 510/6=85 and 85/4=21.25. To sum up, you can increase one stat by 22, and the rest by 21. Here are Treecko's stats, with the average stats included:
Lv100 Treecko
HP: 242
Atk: 147
Def: 127
SAtk: 187
SDef: 167
Spd: 198
As you can see, you are still not even close to the base stats of a Sceptile and we haven't even EV trained the Sceptile yet either. So, since averaging out a stat doesn't work, let's focus our EVs on a particular stat. The limit you can increase EVs on a Poke is 255, which means we can make out 2 stats, which will increase the stat by 63. Let's focus on SAtk and Spd, since that is what the Sceptile family is good at. Here are Treecko's stats again.
Lv100 Treecko
HP: 221
Atk: 126
Def: 106
SAtk: 229
SDef: 146
Spd: 239
Even with the two stats maxed, they STILL aren't as high as our untouched Sceptile. Not to mention the fact that the other stats that weren't maxed are much lower. Okay, last example. Let's cheat again and max out every stat. This is illegal and can only be done by cheating, but this is for arguments sake. So every stat on our Treecko increases by 63:
Lv100 Treecko
HP: 284
Atk: 189
Def: 169
SAtk: 229
SDef: 209
Spd: 239
In case you have forgotten, here is our Lv100 Sceptile that has still not received any EV training:
Lv100 Sceptile
HP: 281
Atk: 206
Def: 166
SAtk: 246
SDef: 206
Spd: 276
End result, our illegal Treecko is only 3 points higher in HP, Def, and SDef, while it is still 17 points short in Atk and SAtk, and a whopping 37 points behind in Spd. You can even give your Poke a beneficial Nature, which will increase one stat by 22 points, but it will also reduce another by 22.
So, as you can clearly see, even a cheated Treecko cannot match up to an untouched Sceptile. You can battle with unevolved Pokes because they are cute or what not, but PLEASE don't fool yourself into thinking that a pre-evolution can in any way be as strong as its fully evolved version.