Yeah, the tree is an optical element for sure, as it's moving over top of the matte painting, but the detail on the live action element at the bottom is VERY nice for a comp, so I thought it might have been packed in-camera somehow. That said, the opticals look surprsingly awesome throughout Ghostbusters on Blu-ray, many of them exhibiting less grain than the non-FX shots. AFAIK they weren't shot on VistaVision, but a quick search of the internets reveals that they were shot large format on 65mm, hence the superior quality. It's even said in this transcribed Starlog article
that Boss Film "actually [had] to degrade the effects shot to match the live action".
So I may not have been wrong about that shot being an optical after all.