Tampa Bay area office vacancy inched up in the first three months of 2008, hurt by a sluggish economy that's left fewer large tenants to go around. Fifteen large move-outs totalling 366,000 square feet lifted vacancy from 11 percent at the end of 2007 to 11.7 percent by March 31, according to real estate company Colliers Arnold. Among the biggest defections was the closing of the 750-employee Home Depot call center in Brandon. Colliers hinted that vacancies could rise further with the construction of 1.1-million square feet of new offices, half in Tampa's Westshore district. Further pressure comes from 1.1-million square feet of vacant space available for sublet, the highest total since 2002.