We have a 3 year old vinyl sided home with a concrete step down foundation. Water appears to be seaping under the sill plate into the basement. This occurs during prolonged or driving rain. Both the contractor and I have caulked with little improvement. My next approach is to fill from the outside with spray foam insulation to seal any and every possible point of water entry. We want to finish the lower level but not until it is dry

There should be a weather barrier (building felt or housee wrap) and it should overlap the foundation.

Sounds like you need to remove some siding to see what is going on.