I couldn't find the schematics for the classic adapter, so I don't really know exactly how the mappings were done. Some are obvious (D2 -> D2) but some could be mapped more than one way, and I don't know how it was done.
Since I am powering photons on 5V via Vin - this should connect to VUSB?
Yes, with one really important caveat. Unlike VIN on the Photon and Electron, there is no diode on VUSB - it's connected directly to the USB connector VBUS so you should never power both the VUSB pin and the USB connector.
In most Feather wings, VUSB is meant to be 5V power from the USB connector into the wing, but you can feed power the other way (with caution).
Enable on Xenon would connect to GND on Photon
The Xenon EN pin determines whether the Xenon powers the 3V3 pin. If low, 3V3 is not powered. If high, the 3V3 is powered by the Xenon and a Feather wing can use 3.3V power between 3V3 and GND
Also, SCK = > A3, MOSI =>A5, MISO=>A4 so presumably Xenon A3, A4, A5 can be either DAC, WKP or NC?
On the Xenon, there are dedicated pin for SCK, MOSI, MISO. For example, SCK is D13, not A3. I'm not sure how the classic adapter maps A3 - A5, it could do so either as analog pins or as SPI pins and it's not clear what was done.
There is no DAC on the nRF52840 so if you need a DAC, you'll need an external I2C or SPI DAC. However presumably A6/DAC maps to A6 on when using the classic adapter as it still can be used for ADC.
The nRF52840 is much more flexible on pin mapping so any pin can be a wakeup pin, not like the STM32Fxxx that require the dedicated WKP pin to wake from deep sleep.
The picture of the classic adapter is wrong. It was a pre-production sample and they accidentally soldered female headers on the bottom. They should be male.