oh, no! most cats metabolize the insulin much quicker, so giving one shot means they'll be without insulin for a significant amount of time.
honestly, a vet who suggests one shot a day for a cat that is not nearing remission does not know the essentials about feline diabetes. mine did the same thing -- suggested i shoot cleo once a day (in his case, for a week) when she started insulin. as soon as i heard that i decided i'd be better off with advice from the lantus forum.
here's a great explanation of the need for twice daily dosing (marvistavet.com has great articles and videos on pet health, so it's a good site to keep bookmarked):
http://www.marvistavet.com/html/body_glargine_insulin.html
other relevant citations:
http://www.vetinfo.com/regulating-feline-diabetes-lantus.html
University of Queensland article:
http://www.uq.edu.au/ccah/docs/diabetesinfo/link5.pdf:
"Better glycemic control is achieved with twice daily dosing rather than once daily"
http://www.uq.edu.au/ccah/index.html?page=43391&pid=0 on page 8: "Studies in healthy cats indicate twice daily dosing provides optimal glycemic control, and studies in diabetic cats also support twice daily use. ... This is also supported by the remission rates obtained in newly diagnosed cats."
The university of queensland article cited above supports twice daily dosing but it discussed the Rand protocol, which involves keeping cats in a clinic while they are on insulin at higher doses that may be unsafe at home. Jacquie Rand, the author, is now a supporter of the tilly protocol, a slightly modified version of which is followed at FDMB's lantus insulin support group, and is working on a study with the german katzen forum that came up with tilly. the german forum and our lantus group at fdmb have extremely encouraging remission rates for cats on lantus.
honestly, lantus works better if you build up to the proper dose in small increments. a large shot once a day into a cat will most likely mean a bigger drop in blood sugar, a bounce up from that drop as the body goes into self-protection mode, likelier hypo (extremely low blood sugar), then a period during which there's no more insulin left to keep blood sugar down. a bounce can mean higher numbers for multiple days.
you'll see here if you check our spreadsheets that even a late shot (at 14 hrs after the prior shot) can lead to higher numbers as the cat runs out of insulin/overlap between shots. shots every 24 hrs only will likely lead to uneven blood glucose control throughout the day, which can damage chances for remission/regulation for the cat.