Having already informed us in Shemos (17:1) that they were in Refidim, why does the Torah see fit to write here "Vayis'u me'Refidim"?
Seforno: It teaches us that they left Refidim with the express intention of arriving at Midbar Sinai. 3
See Ramban's objection to the question. He therefore cites the Mechilta, which bases the question on the Pasuk in Bamidbar (33:15) which, in connection with the journeys and encampments of Yisrael in the desert, includes the one from Refidim to Har Sinai - so why 'repeat' it here?
Why does the Torah write "Vayavo'u ... " and not "Vayachanu ... "?
Ramban: So excited were they, that the moment they arrived they entered the desert and saw Har Sinai in front of them, they encamped on the spot, 1 without searching for a suitable location to set up camp. 2
Why does it add the words "Vayichan Sham Yisrael"?
Ramban #1: To teach us that they separated the Eirev Rav from their midst. 1
Ramban #2: In order to mention them in a dignified manner when they came to receive the Torah.
Seeing as the Torah was given to Yisrael exclusively, as the following Pasuk clearly states (Ramban).
Why "Vayichan" in the singular?
Rashi and Targum Yonasan: 'Like one man, with one heart!' 1
In contrast with all the other encampments, throughout the forty years, which were performed with strife (Rashi, citing the Mechilta).
Rashi writes that they left Refidim in Teshuvah. The Mechilta says that Refidim is an acronym for 'Rafu Yadayim' (they slackened from learning Torah), and this is why Amalek came!
Riva, Moshav Zekenim: When they camped in Refidim, they slackened. When they left, they repented. "Nas'u"