Yes, Nokia 8110 4G is a dual sim mobile phone. You can use one 4G sim card in one slot and another slot will be on the 2G network.
Nokia 8110 4G runs on KaiOS which does not have the WhatsApp support. Nokia management has updated that they are working on the same but there is no definitive promise made for the same.
Yes, Nokia 8110 4G does have the vibration alert.
Nokia 8110 is 133.45 mm ( 13.34 cm) long and 49.3 mm ( 4.93 cm ) thick.