Both words are used to describe people who are very sociable, comfortable, or confident in the company of others. Outgoing, the more common and informal word, describes someone who behaves in a friendly, lively way, is less inhibited than the average person, and readily engages in conversation with others, whether friends or strangers. You’ll find gregarious is often used in much the same way, and it is accepted as a strong, more formal synonym for outgoing. However, in most dictionaries it is defined as “enjoying the company of others,” which is an extension of its other, older definition that refers to animals that live in herds rather than alone. So, strictly following the definition, gregarious doesn’t necessarily imply any particular behavior with others, except the preference to be with others (like a bird in a flock), or to be social.
Both words refer to keeping an action or process going without interruption. With continue, the subject can persist in its own movement (I continued to dance) or keep something else going (They continued the conversation), whereas sustain is limited to the latter case and always takes a noun as its direct object (How did you manage to sustain the conversation?). In terms of meaning, continue emphasizes an activity that is kept up, while sustain implies keeping something in existence or making it last: sustain economic growth; sustain the momentum or pace.
Change refers to an instance of something becoming different than it was or being replaced by something new: a change in leadership; changes in the environment. Vicissitude refers to one of a succession of changes or alterations in the state, form, or condition of something over time. The most familiar use of vicissitude is in the plural, as in the vicissitudes of life. In this sense, the vicissitudes—of life, history, politics, fortune, love, or the market—suggests something like “the twists and turns” and “ups and downs” that characterize the course of any of these major human affairs when considered in its entirety. When we use “change” as a mass noun, as in “Change is constant,” this approaches the meaning of vicissitudes, but is more abstract and general. Vicissitudes evokes the particularity and number of the changes we weather.