Two incredible camera phones with different strengths. Mat Smith for Engadget For some companies, the evolution of smartphone camera tech can't come fast enough. In the last few years, both Vivo and Oppo have been experimenting with teleconverters, barrel lens add-ons that expand the zoom of their flagship phones to dizzying heights. This year, two "ultra" flagships have launched side by side, with similar propositions as true replacements for your dedicated camera. Vivo's X300 Ultra and Oppo's Find X9 Ultra have a lot in common. Both have teleconvertors, yes, but the companies have also allied themselves with storied camera brands; Vivo worked with Zeiss on both lenses and its color technology, while Oppo continues its Hasselblad partnership through the camera software, some hardware design touches and its 10x telephoto camera. Both phones have a lot in common, but what's more interesting are the different choices Vivo and Oppo made with their strongest feature: the cameras.…