"Slick" really is an inappropriate term. But I think Cal Tjader had a more beautiful vibes sound in 1955 than Bags. It's a combination of many factors: the instrument itself, the oscillator setting, the mallets, and your striking technique. It's not just the studio engineer. Listen to Bags on other pre-1955 recordings. I dare say Jackson listened to Tjader and others and changed his sound (didn't he switch his vibes brand around that time?). The smaller labels had only one vibes player in their catalog, and Savoy was looking for one. Bags was lucky that Tjader opted for Fantasy and California, for convenience,