Should I supply CP2102 with power from the BUS pin?
You MUST always connect VBUS to the +5V feed in some way (either directly or through a divider). The IC will not operate otherwise, it uses this signal as an indication that USB is connected.
Or should I feed it with the 3v3 that come from the LDO?
Per the datasheet,
Alternatively, if 3 V power is supplied to the VDD pin, the CP2102/9 can function as a USB self-powered device withthe voltage regulator disabled. For this configuration, it is recommended that the REGIN input be tied to the 3 V net to disable the voltage regulator. In addition, if VDD or REGIN may be unpowered while VBUS is 5 V, a resistor divider (or functionally-equivalent circuit) shown in Note 1 of Figure 8 and Figure 10 is required to meet theabsolute maximum voltage on VBUS specification in Table 2.
No matter what, all of the configurations show at least some connection to VBUS - either directly or through a resisor-divider.
Whenever I disconnect VBus the CP2102 automatically enters on standby mode?
I'm not sure about that, but most likely the USB transceiver would go idle since the chip would recognize that the USB connection is not present.
Should I use any pull-up resistor on D+ and D-? Datasheet doesn't make any reference to that.
This is fundamental USB knowledge, so the datasheet author most likely skipped this in the datasheet. Pullups - no. Depending on your PCB layout, it sometimes does not hurt to put zero-ohm resistors in series with D+ and D- in case you need to tweak the impedances slightly (i.e. if you get data errors with long cables and whatnot, you can change to a few ohms to dampen things). The CP2102 only runs at full speed (12MHz) so it's fairly forgiving in this regard.