Yes of course it is possible to make vegetarian or vegan Pho. In Vietnam these options are increasingly popular. But please check with your restaurant whether the stock used in your vegetarian Pho is actually vegetarian or not as it is common for meat stock to be used as a base for vegetarian Phos in Vietnam.
At MamaPho we currently only provide meat based stocks but we do have vegetarian stir fried options including no meat products.