Can my employer force me to take vacation time while out on disability?

Yes, an employer can require that you use vacation time while out on disability. First, note that unless you are taking some legally protected medical leave--e.g. Family and Medical Leave Act leave--an employer does not have to necessarily even let you take time out for illness; employers may terminate employees (who don't have contracts to the contrary) if they don't show up for work. And if you are using some leave protected by law, like FMLA, you have to make sure that your company is covered (meets the requirements), you are covered (meet the requirements), and that you complied with whatever notice and proof requirements are mandated. Otherwise, it's voluntary whether the company lets you take leave, and being voluntary, they can put limitations or conditions on it.

Second, even if you are using a protected leave, those allow the company to require you to use paid time off (e.g. vacation, sick, personal, or comp time) for some or all of the leave; you get to take the leave and keep your job, but you may have to use your days off. Below, for example, is a link to a relevant page put out by the DOL about FMLA; if you scroll down, you'll this requirement.

Look at it this way--you're not actually losing anything. After all, by using the vacation, you are being paid for part of the leave, where otherwise  you'd not be paid. And an employer could refuse to let you use vacation shortly after your having been out on an extended leave, so you might not otherwise have the chance to use that week for awhile.

Here's the link: http://www.dol.gov/whd/regs/compliance/whdfs28.pdf