good luck with your virginia buttonweed, it will be a battle! depending on the severity of your problem it likely wont be gone with two apps of celsius. my new home had a huge infestation of VB. i had to physically remove on patch of about 60 sq ft. that left nothing but bare soil the mat had grown so thick. what ive done is i applied my yearly max of celsius in two blanket applications in the spring. (one at max label rate and a second 4 weeks later with what was left available to meet the yearly max). i had to spray a few more sprouts with MSM in early summer. (its something i had available and is labeled for suppression of VB). it knocked back what had come up but i have more sprouts popping up in trouble areas in the last couple of weeks that i am going to his with dismiss NXT tonight (again labeled for suppression and available to me, plus targeting other weeds i have coming up.)
all the research i have done has found that with major issues like i have its a multi year battle. that being said celsius did a good job, it took its sweet time doing it but im confident it will take care of it with time. ill do the same thing next year, heavy celsius app in spring and use my other chems if more pop up. my huge problem areas have grown back in all except the biggest patch, its grown in 95%. (ive found other issues hindering the spread of my bermuda in that area)