The essential oils of five Vietnamese Syzygium species (Syzygium levinei, S. acuminatissimum, S. vestitum, S. cumini, and S. buxifolium) were first hydro-distilled and analyzed using GC-FID/MS (gas chromatography–flame ionization detection/mass spectrometry). Monoterpene hydrocarbons, sesquiterpene hydrocarbons, and oxygenated sesquiterpenoids were the main chemical classes in these oils. All these essential oils showed good–excellent antimicrobial activities against Gram-positive bacteria Enterococcus faecalis, Staphylococcus aureus, and Bacillus cereus, and the yeast Candida albicans. S. levinei leaf essential oil, rich in bicyclogermacrene (25.3%), (E)-β-elemene (12.2%), (E)-caryophyllene (8.2%), and β-selinene (7.4%), as well as S. acuminatissimum fruit essential oil containing (E)-caryophyllene (14.2%), α-pinene (12.1%), caryophyllene oxide (10.9%), β-selinene (10.8%), α-selinene (8.0%), and α-humulene (5.7%), established the same MIC value of 8 µg/mL against E. faecalis and B. cereus, which were much better than the positive control streptomycin (MIC 128–256 µg/mL). The studied essential oils showed the potential to defend against mosquitoes since they caused the 24 and 48 h LC50 values of less than 50 µg/mL against the growth of Culex quinquefasciatus and Aedes aegypti larvae. Especially, S. buxifolium leaf essential oil strongly inhibited Ae. aegypti larvae with 24 and 48 h LC50 values of 6.73 and 6.73 µg/mL, respectively, and 24 and 48 h LC90 values of 13.37 and 10.83 µg/mL, respectively. These findings imply that Vietnamese Syzygium essential oils might have potential for use as supplemental antibacterial agents or as “green” alternatives for the control of mosquitoes.