First, there are several different sleep modes. I won't go into them here, but I'd suggest looking up "Hibernate" and "Standby" and "Power Options" to make sure that YOU understand what the differences are. Since you have VISTA, Ill try to work with that.
First, make sure that you are completely up-to-date on your Vista patches. Ss some will say, consider a different OS such as Windows 7, Windows 8 or Linux. Next, scan your computer, usually by looking at your control panel "Add/Remove Programs" section and see if you have any power control software. This includes APC, Tripplite, Dell or some other software that you may think is either power-oriented or maybe laptop-specific software (I admit I didn't check: this is a laptop correct?). If you have anything suspect, please make sure you have the latest versions and that you don't have more than one piece of software dealing with your power that can sleep or shutdown your system. They could interfere with each other.
Now, lets look at settings. When you cold boot (power-on) the system, there is usually a key that you can hit that will put you into "Setup". Find out what that is and go into setup. Look through ALL of the settings and see if there is something power-related, such as "Power Options".
So, what options do you need? First, there are usually two option sets: One for if you are plugged into wall power with either a powercord/adapter or a docking station. The other set of settings will be for when you are running "On battery". By the way, if you use a UPS, do you realize that, for a laptop, you already have a built-in UPS? It's called the battery. If your power goes out, the battery in the laptop will continue to hold power on.
Here are some recommendations:
Use a power scheme or custom scheme that does not shut down anything as long as you are plugged into wall power. The answer to what time to shut down things should be NEVER. There is no reason to. If you worry about your screen or you are really interested in saving electricity, you can always use a screensaver. Or just do a proper shutdown if you are going to be away for awhile. That should cover the mode when you are plugged into the wall power.
For when you run on battery as away from the power cord, you have a little bit of discretion as you might want to save your battery say, on an airline flight. However, you can always shut down as soon as you stop using the machine. One thing I would NEVER do is to shut down the hard disk(s). I just won't do that. Sometimes software just does not want to wait for disks to spin back up.
So, where do you need to make these changes or to check the settings? First, as I said, go into the computers "setup" screen when you boot and look for "power options" and check in there.
After you get into Vista, go to your control panel and look for power options there. Make sure the settings are as you wanted them.
Next, go into your display properties and make sure you visit the screensaver tab. Don't have the screen saver kick on too quickly. Also, under screen saver there should/may be "power options" there to. Make sure you confidently make sure everything is in sync.
As DXJANIS said, change your setting to not put the laptop in sleep mode when you close the lid.
Finally, remember that when you do go into a sleep mode, it sometimes takes awhile to wake up (especially if your disks are not spinning). While some sleep modes can wake the laptop when you hit the "enter" key, many sleep modes require you to push your power button momentarily. If you hold too long, your machine will instantly shut down. If you need to do this, you will probably be looking at a couple of minutes for the drive to load up and for the image to go back into memory.
Another thing to verify is that your system clock is working. For a few times, make sure that the windows time is correct. A thing you might want to replace is the "CMOS battery". It is usually a small bettery on the mother board. Check your system documentation to see if you have one and how to replace it.
Finally, you may want to test the main battery on a laptop. Maybe it is not holding a charge or if your charger is not working. If all of your power opions say to NEVER turn things off when plugged into the wall and things are turning themselves off, maybe the system is not registering that the charger is indeed working.