Even though the train is sold out, since you are boarding in DC (which is the origination point of the Vermonter), you should get seats together.

Even if you did not board at the origin (say you boarded in Baltimore) and the train is sold out, you may have to set apart but someone may be getting off in Wilmington, and then there will be seats together! Also, many pax are kind enough that if you ask if they could move so you and your wife/husband/child/etc... could sit together, they may just let you. Also, many car attendants will attempt to seat a group together!

Unlike a plane, you will not have assigned seats (such as 21 A & B) before you board. You can take any available seat that is open. The conductor after (s)he collects your ticket(s) places a "seat check" above your seat. Most times you can move to another seat - just take your seat check with you! (That shows that your ticket has been collected and that the seat is occupied. It also shows where you are getting off, so when the conductor comes through before the station, (s)he can say "____ is the next stop in ___ minutes" and give you time to prepare to leave.)