TCA9517A works as a I2C buffer between master and slave, is it ok to power VCCA and VCCB by a same 5V rail?

Hi May,
Yes. VCCA and VCCB can be tied to the same 5 V rail. In the datasheet we recommend that VCCA should never be higher than VCCB. So please make sure that this condition does not happen.

Thanks,
Rajan