I've always maintained that the eyewear is made of a transparent lens, similar to glasses and contacts of today. So, you can use your native vision to see through them without penalty. It is transparent to all metahuman visible wavelengths (thermo, low-light, and normal vision). This is completely passive.
On top of this is placed a projection surface - possibly holographic in nature and projected outside the device or simply a slightly reflective surface so that an image is projected on the device. However it is achieved, it does not interfere with the transparent properties of the lens. If you then add imagelink, it can display images from external devices, otherwise it only displays overlays from the vision enhancements that are integrated into that device.
So, if you have contacts with thermo, you can see using your normal vision (and low-light if you have it, and thermo if you have it, but why do your contacts have it in that case?) and the thermo from the contacts is optionally overlaid on the "screen". You cannot overlay AR or info from team-mates or other sensors since the contacts do not have the image link accessory.
Technically, I think that if you have DNI, image link is not necessary as it allows you to see AR (which includes things like sensor overlays) without additional hardware. But that depends on how particular your GM is.
ETA: some minor edits for clarity