Is seeing a different girl in a dream, other than the one for whom the Istikhara prayer was performed, and her father is a drug addict, considered a sign after the Istikhara prayer?

Istikhara (seeking guidance from Allah) is recommended for permissible matters. A vision is not a prerequisite after performing it; rather, the preponderant view is that the person who performed Istikhara should proceed with the matter unless Allah diverts them from it.

The vision you mentioned has no bearing, and you can proceed with the engagement. The corruption of the righteous girl's family does not harm her, as Allah Almighty says: "And every soul earns not [anything] except for itself, and no bearer of burdens will bear the burden of another."