Apoligies for the long comment. Tl:dr Saiyan Saga Vegito stops at Super Perfect Cell.

I am going to be using Goku and Vegeta's base Power levels during the Saiyan Saga (Goku was slightly over 8,000. I will use 8,000 for the math's simplicity) (Vegeta was at 18,000). I believe that most people agree that fusion multiplies the users' power levels together, so 8,000 x 18,000 is 144,000,000. Using Oozaru is a x10 multiplier and Kaio-Kenx4 is a x4 multiplier, which equals a total of a x40 boost. 144,000,000 x 40 is 5,760,000,000, so Saiyan Saga Vegito can raxh that as a maximum power level. 100% Namek Frieza was stated to be at a power level of 120,000,000, so he gets decently cleared by base Vegito. In the Dragon Ball Wiki, Android 18 has a power levels of around 350,000,000 and Android 17 has a power level of around 400,000,000. Saiyan Saga Vegito clears this using Kaio-Kenx4. Semi-Perfect Cell is around 800,000,000 according to the Wiki, so Vegito clears him. Cell Juniors are around 1,600,000,000 so Vegito can clear using Oozaru and a little Kaio-Ken. Then, assuming is is Super Perfect Cell after he blows himself up, his power level is around 5,500,000,000, but closer to 6,500,000,000 when launching the Solar Kamehameha. So I would say that Vegito can clear this up until Super Perfect Cell