Most integrated circuits have things called 'clamping diodes' on all the inputs. They're basically diodes that are reverse-biased under normal operating conditions, but start to conduct if the input moves too far past the supply rails.
In most cases, taking a pin's input more than .6v above the Vcc pin will make the upper diode start working, and taking the input more than .6v below GND will make the lower diode start working. The upper diode is connected to the chip's internal Vcc rail, so you can actually power a chip through any of its I/O pins when the VCC pin is disconnected. I won't say it's a good idea -- the clamping diodes aren't made to handle a lot of current -- but it's physically possible.
When you connect your 3.3v supply, the VCC pin will take over again. Just remember to keep the input signals below 3.3v.
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