Things will have undoubtedly have changed between then and now, but I had an opportunity to ascend the Bitexco Tower in February of 2012, not 18 months after it was opened as the then-tallest building in all of Vietnam.
The little protruding platform you see in the images is the visitor's site, the Skydeck. Offering panoramic but not quite 360-degree views of the sprawling city below.
Opening hours for the Skydeck are long, meaning you can go in the morning through evening and into the night most days of the week. You're not allowed to bring much with you, including a total ban on chewing gum. Don't plan it if you're intoxicated or plan to be, Vietnam's beer is tasty and the bars around Ho Chi Minh are plenty and welcoming.
You could probably pick up tickets on the day of arrival, but it wouldn't hurt to plan ahead if it's what you'd like to see or do during your visit. It's currently priced at a whopping 200,000 Vietnamese Dong for adults, 130,000 for children...
That's about US$9 for adults and about US$6 for kids.