Question
are there vegetables that cant or shouldnt be planted side by side??

Answer
Ivan, there are many plants that benefit from being planted next to one another, and many that should never be planted together.  This is referred to as companion gardening.

Many plants are considered companions to others because they either aid their growth or deter pests.  Examples would include basil with tomatoes; marigolds with beans; onions with anything except legumes (beans and peas); dill with cabbage and broccoli; corn with beans and cucumbers; and radishes with squash.

The reason companion planting is so effective is because the smells of the various herbs, flowers, and vegetables confuse and/or repel the insects. Dill is a perfect example of an herb that repels the cabbage moth, therefore making it a perfect companion for members of the brassica family
such as cabbage, broccoli, and cauliflower.  Marigolds are a good example of a flower that not only repels through scent, but also releases a substance through its roots that kill nematodes in the soil.

Just as there are plants that thrive next to one another, there are many plants that do poorly when grown in close proximity to certain plants.

Examples would be peas or beans planted near garlic or onions.  Another one is carrots and dill.  When planted within 10 feet of one another, the vegetables suffer from deterred growth and production.

A good website that will give you quite a bit of information on companion planting is:

http://www.gardenguides.com/TipsandTechniques/vcomp.htm

A good reference book is "Carrots Love Tomatoes" by Louise Riotte.  

I hope this information helps.  Please write again if you ever have any other questions.

Regards,

Mike