Can i use Zyxel vmg3326-d20a vdsl for connecting o2 dsl ?

I can't even find a data sheet for that device. Make sure that it supports VDSL2 vectoring (G.993.5). According to the interface specification you have to use VLAN-ID=7 on the WAN port.

Whenever you decide to use your own modem-router: Configuration is your job, not that of o2.
Please make sure that the device you have selected meets the interface description. If you need device specific help, ask the supplier of your device.

Ok thanks. If i plan to buy o2 home box which model is better to go ? 6641 or 6441 ?

You should not buy any of them, lease them from O2 directly.

The 6641 has an ISDN S0 bus, the 6441 uses pots.

You can not buy an O2 Homebox. If you decide to rent an O2 Homebox from O2, you can not choose the exact modell. Usually you get an O2 Homebox 6741. If you ordered the option "ISDN Komfort" (3 phone numbers instead of 1), you should get an O2 Homebox 6641.

Please be aware that the user interface of all O2 Homebox supports only German language. As these devices are designend for simple tasks, make sure the it has all the features you want to use.

I tried to use a friend's o2 box... but the PIN activation is not possible. Someone from o2 could help with that?

You can't use a foreign or bought O2 Box at another connection. You have to get your Box from O2 directly.

And if you have a Flex tariff better buy a router by yourself and don't rent from O2. You must pay one-time 50Euro in Flex tariff.