And do I smile, such cordial light Upon the Valley glow – It is as a Vesuvian face Had let its pleasure through –
This stanza describes the speaker smiling.
One way to paraphrase this is, "And if I smile, it’s like an uptight hot-head had actually loosened up a bit."
The smile is cordial, and shines upon the Valley, but it’s like a "Vesuvian face" had smiled.
("Vesuvian" references Mount Vesuvius, a volcano in Italy. Mount Vesuvius erupted in 79 A.D. and buried the city of Pompeii and many of its inhabitants. Not a pretty picture. To say someone is Vesuvian is to say that they’re prone to explosive anger.)
So, the speaker smiling is like a Vesuvian face. In other words, we have the image of a person with anger-management problems, who cools down a bit and "let its pleasure through."