Why does the plot summary of Villette say that the setting is Belgium and then go on to say: "Tiring of her humdrum existence, she went to France"?
Lucy Snowe went directly to Belgium (Labassecour) because that was all she could afford. She then traveled the forty miles from the port of her arrival to Villette (Brussels).
Which plot summary are you refering to? You are completely right. In Chapter Six of this excellent novel by Charlotte Bronte, which is actually loosely based on the author's own life, Lucy Snowe, fleeing from some unspecified family disaster, having arrived in London, then determines to engage in a risky endeavour. Note how she justifies taking such a step:
My state of mind, and all accompanying circumstances, were just now such as most to favour the adoption of a new, resolute, and daring--perhaps desperate--line of action. I had nothing to lose... If I failed in what I now designed to undertake, who, save myself, would suffer? If I died far away from--home, I was going to say, but I had no home--from England, then, who would weep?
Having "nothing to lose" and being friendless, alone and without contacts in England, she determines to travel to Belgium and seek work and a new life there. Thus it is, as you state, that she travels to a port in Belgium and then overland to Villette. Maybe you were looking at a different plot summary or perhaps the plot summary itself was erroneous. If you want to include the link to the plot summary we might be able to help you more.
The Salem series of critical essays doesn't list all components of a work's settings, just the predominant one. In this case, that is seen to be Belgium where the story starts.
Thank you, accessteacher. Here is the link to the summary in which I saw the error. It's probably a typo but it should be corrected.