Note: you have to make it to the secret place with full life.
Level Passwords Introduction 3721 1281 3751 4456 Gravity Beetle 5623 4888 5851 4221 Blast Hornet 1745 5231 5441 2486 Neon Tiger 3621 4867 5851 2227 Tunnel Rhino 5728 1263 5754 2458 Blizzard Buffalo 7671 2857 2144 1247 Volt Catfish 1778 5253 2444 3488 Crush Crawfish 5718 1266 2727 7458 Dr. Doppler's Lab 5718 1263 2627 7458
If you have all of thee-tanks, weapons, hearts, and full energy you'll find a capsule that will grant you the four Super Adaptions.
The first entrance is at the beginning of the Volt Catfish stage. A platform will take you to Dr. Vile.
The second entrance is in the Frozen Buffallo stage. It is located in the first pit after you enter the small room with two doors. You will see the entrance on the left side.
The third entrance is in the crab's stage (I forget his name.) The entrance is located next to an energy capsule after you leave the small room where you fought a mid-boss. Before you reach the energy capsule you will fall right through the floor. Here you'll find the capsule that takes you to Vile's secret stage.
Note: These entrances are only there after you kill at least two bosses and BEFORE you kill Bit and Byte (the two mid-bosses who talk to you.)
If you'd rather earn it yourself, do this. On the second stage of Doppler's lab play through the whole level with Megaman only. When you get to the mini-boss's layer, switch to Zero. Play through the entire level and defeat Vile. Zero will be damaged beyond repair and give Megaman his saber.