The previous example had 'Je vais en France' --- I am going to France. This example gives 'Je vais à Paris demain' -- I am going to Paris tomorrow
Why is it that we use 'en France' in the previous example and 'à Paris ' in this example , both to denote that we are going TO that destination? can both methods be used?
There is a lot for you here: http://french.about.com/library/weekly/aa062400.htm
"À" with cities: "à Paris", "à Orléans", "à New York", "à Athènes".
"En" with all feminine countries and the ones beginning with a vowel: "en Angleterre", "en France", "en Espagne", "en Iran", "en Uruguay".
"Au" with masculine countries beginning with a consonant: "au Brésil", "au Mexique", "au Sénégal", "au Maroc".
"Aux" with plural countries: "aux États Unis", "aux Pays-Bas", "aux Maldives".
They use different forms depending on city, state, country and continent also gender. En is for going TO a place that is a feminine country and à is for cities i don't think that gender matters for cities, but i could be wrong on that part. His link probably leads you to a graph or examples of the different usages depending on terms i listed above.
This type of liaison between a verb and a preposition is technically allowed, but is apparently very rare / denotes a high register. https://www.lawlessfrench.com/pronunciation/optional-liaisons/