Extra day in Bangkok or Vientiane?

I’m leaving for a near 2 month long trip to SE Asia. Just got an email saying my flight to Vientiane was moved to later in the day due to issues and I got the offer to move the flight to a new day and time free of charge.

I was originally only gonna spend one day/night in Vientiane to catch the train to Vang Vieng the next day and figured I’d look around the city, but based off reviews, Vientiane seems quite bland to most

Would you skip staying in Vientiane and take the extra day in Bangkok? I would fly to Vientiane in the morning and take the train later in the evening in this case